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  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Towards Customizing Gameful Systems by Gameful Design Elements</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Gustavo F. Tondello</string-name>
          <email>gustavo@tondello.com</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Lennart E. Nacke</string-name>
          <email>lennart.nacke@acm.org</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>HCI Games Group, University of Waterloo</institution>
          ,
          <country country="CA">Canada</country>
        </aff>
      </contrib-group>
      <pub-date>
        <year>2018</year>
      </pub-date>
      <fpage>101</fpage>
      <lpage>109</lpage>
      <abstract>
        <p>Recently, several researchers have suggested that personalized gameful systems can be more effective than generic approaches. However, there is still scarce empirical evidence that the suggested factors for personalization, such as gender, age, user types, and personality traits, will be effective in improving user engagement and performance for personalized gameful systems. In this work-in-progress, we present a research plan for empirical evaluation of a customizable gameful system. Upon completion of this study, we expect to provide empirical evidence that the participants' selection of gameful design elements in a practical application will correspond to the theorized relationships suggested by prior survey-based research, and that the system can suggest the gameful design elements that users are more likely to enjoy. The results of this research will provide an actionable path for gamification designers to implement personalized gameful systems and for researchers to develop recommendation algorithms for gamification.</p>
      </abstract>
      <kwd-group>
        <kwd>Gamification</kwd>
        <kwd>Gameful Design</kwd>
        <kwd>Personalization</kwd>
        <kwd>Customization</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>-</title>
      <p>
        Gamification, the use of game design elements in non-game contexts [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ], can be
employed as a toolset to increase user engagement, activity, and enjoyment of digital
interactive systems. It can also be used to create applications aimed at promoting
behaviour change in domains such as health, wellness, education, training, online
communities, customer loyalty, and marketing [
        <xref ref-type="bibr" rid="ref2 ref3 ref4 ref5">2–5</xref>
        ], thus representing a form of
persuasive technology (PT). Recently, a topic that has gained attention is
understanding how to personalize gameful systems to each user [
        <xref ref-type="bibr" rid="ref6 ref7 ref8">6–8</xref>
        ]. This is important because
personalized interactive systems can be more effective than generic systems [
        <xref ref-type="bibr" rid="ref8 ref9">8, 9</xref>
        ].
Gameful systems are effective when they help users achieve their goals, which often
involve educating them about certain topics, supporting them in attitude or behaviour
change, or engaging them in specific topics [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]. However, publications on
personalized gamification so far have been mostly theoretical, for example, focusing on
identifying different personality traits [
        <xref ref-type="bibr" rid="ref10 ref11">10, 11</xref>
        ] or preferences for personalization [
        <xref ref-type="bibr" rid="ref12 ref9">9, 12</xref>
        ].
      </p>
      <p>
        In work-in-progress this paper, we present a research plan for empirical evaluation
of a customizable gameful system. Our design approach includes understanding the
different user preferences based on the Hexad framework [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ], then allowing users to
select the gameful design elements [
        <xref ref-type="bibr" rid="ref14">14</xref>
        ] that might be most appealing to them.
Moreover, a gameful system might try to identify the user’s preferences and suggest which
elements they are more likely to enjoy [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ], akin to what recommender systems (RS)
[
        <xref ref-type="bibr" rid="ref16">16</xref>
        ] do in application domains such as online commerce. Therefore, this research
aims to investigate empirically if this kind of tailored gameful system is more
engaging to users than generic implementations.
2
2.1
      </p>
    </sec>
    <sec id="sec-2">
      <title>Related Work</title>
      <sec id="sec-2-1">
        <title>Gamification User Types</title>
        <p>
          Research on gameplay motivations has shown that players have diverse personal
preferences regarding how and what they play [
          <xref ref-type="bibr" rid="ref17 ref18 ref19">17–19</xref>
          ]. Researchers have developed
player type models [
          <xref ref-type="bibr" rid="ref17 ref20 ref21">17, 20, 21</xref>
          ] or gamer motivation scales [
          <xref ref-type="bibr" rid="ref18 ref22">18, 22</xref>
          ] to capture the diverse
styles of play exhibited by different players. This information has been increasingly
used in gamification to model user behaviour and design more engaging gameful
systems. Nevertheless, none of these models have studied elements used specifically
in gameful design. Therefore, their applicability in gamification has not been
supported by empirical evidence yet.
        </p>
        <p>
          To address this problem, Marczewski [
          <xref ref-type="bibr" rid="ref23">23</xref>
          ] developed the Gamification User Types
Hexad framework, based on research on human motivation, player types, and
practical design experience. He also suggested different game design elements that may
support different user types [
          <xref ref-type="bibr" rid="ref24">24</xref>
          ]. The six Hexad user types are [
          <xref ref-type="bibr" rid="ref13 ref23">13, 23</xref>
          ]:
 Philanthropists are motivated by purpose. They are altruistic and willing to give
without expecting a reward.
 Socialisers are motivated by relatedness. They want to interact with others and
create social connections.
 Achievers are motivated by competence. They seek to progress within a system by
completing tasks or prove themselves by tackling difficult challenges.
 Free Spirits are motivated by autonomy, meaning freedom to express themselves
and act without external control. They like to create and explore within a system.
 Players are motivated by extrinsic rewards. They will do whatever to earn a
reward within a system, independently of the type of the activity.
 Disruptors are motivated by the triggering of change. They tend to disrupt the
system either directly or through others to force negative or positive changes.
Moreover, Tondello et al. proposed a validated survey measure [
          <xref ref-type="bibr" rid="ref13">13</xref>
          ] for scoring
individuals across these user types. They also suggested that the Hexad can be used as a
model to personalize user experience (UX) in gameful systems, by showing that there
are significant correlations between the Hexad user types and user preferences for 32
design elements commonly employed in gameful design [
          <xref ref-type="bibr" rid="ref13">13</xref>
          ]. Orji et al. [
          <xref ref-type="bibr" rid="ref25">25</xref>
          ] further
supported this suggestion by also showing significant relationships between the
Hexad user types and the persuasiveness of different strategies commonly employed in
persuasive technologies.
        </p>
      </sec>
      <sec id="sec-2-2">
        <title>Personalized Gamification</title>
        <p>
          Regarding models of user preferences, Ferro et al. [
          <xref ref-type="bibr" rid="ref10">10</xref>
          ] studied several models of
personality and player types, aiming to find the similarities between them as well as to
relate them to different game design elements. Their work grouped personality traits,
player types, and game elements in five player categories: ‘Dominant’, ‘Objectivist’,
‘Humanist’, ‘Inquisitive’, and ‘Creative’. Jia et al. [
          <xref ref-type="bibr" rid="ref11">11</xref>
          ] studied the relation between
the five-factor model (FFM) personality traits [
          <xref ref-type="bibr" rid="ref26">26</xref>
          ] and individual gamification
elements and found several significant correlations. Orji et al. [
          <xref ref-type="bibr" rid="ref9">9</xref>
          ] studied the relation
between the FFM personality traits and several persuasive strategies used in
gamification and found significant correlations.
        </p>
        <p>
          Gamification also draws from research in persuasive technologies to further
encourage adoption of behaviours. Theoretical and empirical studies have suggested
different factors for persuasive technology personalization [
          <xref ref-type="bibr" rid="ref27 ref28">27, 28</xref>
          ], such as
personality types [
          <xref ref-type="bibr" rid="ref29 ref30 ref31">29–31</xref>
          ], age [
          <xref ref-type="bibr" rid="ref32">32</xref>
          ], gender [
          <xref ref-type="bibr" rid="ref32 ref33">32, 33</xref>
          ], player types [
          <xref ref-type="bibr" rid="ref34 ref35">34, 35</xref>
          ], culture or
nationality [
          <xref ref-type="bibr" rid="ref36 ref37">36, 37</xref>
          ], and individual susceptibility to persuasive attempts [
          <xref ref-type="bibr" rid="ref38 ref39">38, 39</xref>
          ].
        </p>
        <p>
          Considering the topic of gameful design elements, Tondello et al. [
          <xref ref-type="bibr" rid="ref14">14</xref>
          ] proposed a
new conceptual framework for classifying them based on participants’ self-reported
preferences, with the goal of understanding user behaviour in gamification. Their
work classified gameful design elements in eight groups [
          <xref ref-type="bibr" rid="ref14">14</xref>
          ]:
 Socialization: elements corresponding to some form of social interaction,
including both collaborative, competitive, and entirely social interactions.
 Assistance: elements corresponding to the user receiving some sort of aid for their
progression, either from the system or from other users.
 Immersion: elements related to immersion and curiosity, including elements
related with a narrative or story or with exploration and unpredictability.
 Risk/Reward: elements related to challenges, gambling, and the rewards that come
from winning.
 Customization: elements related to three different ways of customizing one’s own
experience: (1) customizing the user’s avatar or experience, (2) automatic
personalization, or (3) redeeming freely chosen goods with virtual currency or points.
 Progression: elements related to progression and meaning, representing the will to
be involved in meaningful goals and feeling a progression towards achieving them.
 Altruism. elements corresponding to diverse ways of making a useful contribution,
either to the system or to other users, including sharing knowledge or goods,
contributing to improve the system, and collaborating with other users.
 Incentive. elements corresponding to incentives or rewards that the user might
receive, such as badges, achievements, collectible items, and rewards.
        </p>
        <p>
          It is also noteworthy that despite the existing literature on user preferences in
gamification and games, most gameful design methods do not take user preference in
consideration as part of their process [
          <xref ref-type="bibr" rid="ref40 ref41">40, 41</xref>
          ]. Nonetheless, Ferro [
          <xref ref-type="bibr" rid="ref42">42</xref>
          ] has recently
developed Gamicards, a methodology that helps designers create gameful experiences
by selecting game elements and mechanics tailored to the users and context of the
application.
        </p>
        <p>
          On the topic of customizing or personalizing the activities and gameful design
elements available for the user, three promising approaches have been recently
presented. Khoshkangini et al. [
          <xref ref-type="bibr" rid="ref43">43</xref>
          ] described and conducted an initial evaluation of an
automated challenge generator, which is able to dynamically generate personalized
challenges from templates, by tailoring the goals, difficulty, and rewards according to the
user’s preferences and skills. Altmeyer et al. [
          <xref ref-type="bibr" rid="ref44">44</xref>
          ] and Lessel et al. [
          <xref ref-type="bibr" rid="ref45">45</xref>
          ] also described
and conducted an initial evaluation of a “bottom-up” gamification approach, in each
users are given choices of available gameful elements, which they can customize by
selecting their preferred elements and adjusting some parameters (such as the amount
of points rewarded by an activity). Furthermore, Tondello et al. [
          <xref ref-type="bibr" rid="ref15">15</xref>
          ] suggested
developing recommendation algorithms to suggest gameful activities, gameful elements,
and persuasive strategies that each user is more likely to enjoy in a gameful system.
        </p>
        <p>
          Finally, Böckle et al. [
          <xref ref-type="bibr" rid="ref7">7</xref>
          ] have recently presented a systematic literature review of
the existing approaches on adaptive gamification.
3
        </p>
      </sec>
    </sec>
    <sec id="sec-3">
      <title>Research Plan</title>
      <p>
        Based on the related work on personalized gamification, we have reason to expect
that a customized system will be more engaging for its users and might be better able
to help the user achieve higher performance in the tasks carried out within the system.
For example, Altmeyer et al. [
        <xref ref-type="bibr" rid="ref44">44</xref>
        ] and Lessel et al. [
        <xref ref-type="bibr" rid="ref45">45</xref>
        ] provided initial evidence that
letting users customize their experience—by letting them select the game elements for
their experience—can lead to better engagement and performance. However, if a
gameful system is built with enough activities and gameful design elements to be
appealing to all types of users, a problem of information overload may occur. The
user might find so many different ways of interacting with the system that it might
become difficult to choose their preferred style among them [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ]. One of the existing
solutions for this problem is the use of recommender systems (RS) [
        <xref ref-type="bibr" rid="ref16">16</xref>
        ], which are
software tools and techniques that provide suggestions for particular items to a user.
A RS can help the user find items that would probably match their preferences among
the increasing amount of available information and products. Additionally, a RS relies
on people making choices based on what other people recommend.
      </p>
      <p>
        Although recommender systems can be a solution to tailor gameful systems to each
user, the topic has been scarcely investigated until now. As an initial development of
this idea, Tondello et al. [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ] proposed a general framework that describes the
possible inputs for this type of RS (items, users, transactions, and contextual information),
the possible approaches to choose a recommendation algorithm, and the process
output (the predicted ratings for each gameful activity per user). Nonetheless, the
currently available knowledge on personalized gamification limits the current
framework. Particularly, we still have scarce empirical evidence that the suggested models
for personalization, such as user types and taxonomies of gameful design elements,
will be effective to help users select their preferred activities within gameful systems.
      </p>
      <p>
        To overcome this shortcomings in the literature, we will conduct an experimental
study aimed at answering the following research questions:
RQ1. If allowed to choose the gameful design elements they prefer, do user choices
correspond to the theoretical relationships with user types, personality, gender,
and age reported in previous survey-based studies [
        <xref ref-type="bibr" rid="ref13 ref14">13, 14</xref>
        ]?
RQ2. Do user engagement and performance improve if the application helps them
customize their gameful experience by suggesting the gameful design elements
they are more likely to enjoy (based on the answer to RQ1)?
3.1
      </p>
      <sec id="sec-3-1">
        <title>Study Design</title>
        <p>
          We will build an online crowdsourcing platform in which participants will be asked to
complete classification and brainstorming microtasks. Each task will consist in listing
all the classification tags that the participant can think of for a stock image.
Participants will create an account and will be encouraged to complete as many microtasks
as they wish during the study period. The use of classification microtasks was already
reported on previous studies of customizable gamification [
          <xref ref-type="bibr" rid="ref44 ref45">44, 45</xref>
          ]; therefore, this is
an interesting type of task to allow for comparisons with previous results. Moreover,
brainstorming tasks have also been used in previous empirical studies of gamification
[
          <xref ref-type="bibr" rid="ref46">46</xref>
          ] because they were found to be good types of tasks to investigate task
performance in relation to goal setting. Hence, we will be able to implement gameful design
elements that motivate participants in two levels: (1) to complete more microtasks and
(2) to perform better in each task by listing a higher number of tags.
        </p>
        <p>
          The crowdsourcing platform will include gameful elements to motivate and
encourage participants to complete more microtasks and to perform better in each task.
To allow users to customize their gameful experience, we will include two elements
from each one of the eight groups from our previous classification [
          <xref ref-type="bibr" rid="ref14">14</xref>
          ]. This will give
users a broad range of experiences to select from. From the 16 available gameful
design elements, each participant will be allowed to select up to four elements to
customize their experience. This limitation is added to ensure that users will have to
spend some time selecting the elements that they prefer. The 16 gameful design
elements currently planned for the system design are:
 Socialization: leaderboards and social competition
 Assistance: glowing choice and beginner’s luck
 Immersion: Easter eggs and theme
 Risk/Reward: lotteries and challenges
 Customization: avatars and points
 Progression: levels and progress feedback
 Altruism: knowledge sharing and gifting
 Incentive: badges and rewards
The study will be divided in two phases:
First Phase. The goal of the first phase is to answer RQ1. Therefore, all participants
will be allowed to choose any gameful design element from the list, without any
suggestion from the platform. By doing this, we will be able to verify if participants’
choices will correspond to the theorized preferences by user types, personality traits,
gender, and age reported by the previous studies.
        </p>
        <p>Second Phase. The goal of the second phase is to answer RQ2. Therefore, we want to
test if it is useful for the platform to suggest the gameful design elements that each
user is more likely to enjoy aiding them in their customization. For this purpose, we
will split participants into three conditions:
C1: Tailored: in this condition, the application will suggest the four gameful
elements that the user will be more likely to enjoy based on their profile.
C2: Contra-tailored: this is the opposite of C1; thus, the application will suggest the
four elements that the user is less likely to enjoy.</p>
        <p>C3: Control: in this condition, participants will not receive any suggestion from the
platform regarding their selection of gameful design elements.</p>
        <p>
          Measurements. During the first phase, we will record each participant’s choice of
gameful design elements. Therefore, we will analyze if the independent variables
(gender, age, user type, and personality traits) can predict the participants’ choices for
the gameful design elements and if the relationships between them correspond to the
theorized relationships from previous works [
          <xref ref-type="bibr" rid="ref13 ref14">13, 14</xref>
          ]. In the second phase, we will
measure participants’ engagement (with measures such as the number of completed
tasks) and performance (average number of tags identified for each microtask
completed). Moreover, to better understand the user experience with the customization of
their gameful system, we will also include a few additional free-text questions, which
will focus on their impressions about the activity of selecting gameful design
elements and their general enjoyment of the platform.
4
        </p>
      </sec>
    </sec>
    <sec id="sec-4">
      <title>Conclusion</title>
      <p>
        In this work-in-progress paper, we have described our research plan for an
experimental study aimed at demonstrating the viability of design customizable gameful
interactive systems according to user preferences. Upon completion of this study, we
will be able to provide two main contributions to the extant literature on personalized
gameful systems. First, we expect to provide empirical evidence that the participants’
selection of gameful design elements in a practical application will correspond to the
theorized relationships suggested by prior survey-based research [
        <xref ref-type="bibr" rid="ref13 ref14">13, 14</xref>
        ]. Second, we
expect to provide empirical evidence that it is possible to implement a simple system
to help users overcome the information overload problem, by suggesting the gameful
design elements that they are more likely to enjoy based on their user types and
demographic information. The results of this research will provide an actionable path for
gamification designers to implement personalized gameful systems. Furthermore, the
empirical evidence that will be collected as part of this research will represent a
valuable model, which in the future could be used to implement recommendation
algorithms for gameful systems [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ].
      </p>
    </sec>
    <sec id="sec-5">
      <title>Acknowledgments</title>
      <p>This research received funding from the CNPq, Brazil, the University of Waterloo,
NSERC (RGPIN-418622-2012), SSHRC (895-2011-1014, IMMERSe), CFI (35819),
and Mitacs (IT07255).</p>
    </sec>
  </body>
  <back>
    <ref-list>
      <ref id="ref1">
        <mixed-citation>
          1.
          <string-name>
            <surname>Deterding</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Dixon</surname>
            ,
            <given-names>D.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Khaled</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Nacke</surname>
            ,
            <given-names>L.E.</given-names>
          </string-name>
          : From Game Design Elements to Gamefulness: Defining “Gamification.”
          <source>In: Proceedings of the 15th International Academic MindTrek Conference</source>
          . pp.
          <fpage>9</fpage>
          -
          <lpage>15</lpage>
          . ACM, Tampere, Finland (
          <year>2011</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref2">
        <mixed-citation>
          2.
          <string-name>
            <surname>Seaborn</surname>
            ,
            <given-names>K.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Fels</surname>
            ,
            <given-names>D.I.</given-names>
          </string-name>
          :
          <article-title>Gamification in theory and action: A survey</article-title>
          .
          <source>Int. J. Hum. Comput. Stud</source>
          .
          <volume>74</volume>
          ,
          <fpage>14</fpage>
          -
          <lpage>31</lpage>
          (
          <year>2014</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref3">
        <mixed-citation>
          3.
          <string-name>
            <surname>Nacke</surname>
            ,
            <given-names>L.E.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Deterding</surname>
            ,
            <given-names>S.:</given-names>
          </string-name>
          <article-title>The maturing of gamification research</article-title>
          .
          <source>Comput. Human Behav</source>
          .
          <volume>71</volume>
          ,
          <fpage>450</fpage>
          -
          <lpage>454</lpage>
          (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref4">
        <mixed-citation>
          4.
          <string-name>
            <surname>Raftopoulos</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Walz</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Greuter</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          :
          <article-title>How enterprises play : Towards a taxonomy for enterprise gamification</article-title>
          .
          <source>In: DiGRA</source>
          <year>2015</year>
          :
          <article-title>Diversity of Play</article-title>
          . pp.
          <fpage>1</fpage>
          -
          <lpage>17</lpage>
          (
          <year>2015</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref5">
        <mixed-citation>
          5.
          <string-name>
            <surname>Johnson</surname>
            ,
            <given-names>D.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Deterding</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Kuhn</surname>
            ,
            <given-names>K.</given-names>
          </string-name>
          -A.,
          <string-name>
            <surname>Staneva</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Stoyanov</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Hides</surname>
            ,
            <given-names>L.</given-names>
          </string-name>
          :
          <article-title>Gamification for health and wellbeing: A systematic review of the literature</article-title>
          .
          <source>Internet Interv</source>
          .
          <volume>6</volume>
          ,
          <fpage>89</fpage>
          -
          <lpage>106</lpage>
          (
          <year>2016</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref6">
        <mixed-citation>
          6.
          <string-name>
            <surname>Busch</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Mattheiss</surname>
            ,
            <given-names>E.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Marczewski</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Hochleitner</surname>
            ,
            <given-names>W.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Lankes</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Nacke</surname>
            ,
            <given-names>L.E.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Tscheligi</surname>
            ,
            <given-names>M.:</given-names>
          </string-name>
          <article-title>Personalization in Serious and Persuasive Games and Gamified Interactions</article-title>
          .
          <source>In: Proceedings of the 2015 Annual Symposium on Computer-Human Interaction in Play - CHI PLAY '15</source>
          . pp.
          <fpage>811</fpage>
          -
          <lpage>816</lpage>
          . ACM (
          <year>2015</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref7">
        <mixed-citation>
          7.
          <string-name>
            <surname>Böckle</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Novak</surname>
            ,
            <given-names>J.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Bick</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          :
          <article-title>Towards Adaptive Gamification: a Synthesis of Current Developments</article-title>
          .
          <source>In: Proceedings of the 25th European Conference on Information Systems (ECIS)</source>
          (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref8">
        <mixed-citation>
          8.
          <string-name>
            <surname>Mattheiss</surname>
            ,
            <given-names>E.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Busch</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Tondello</surname>
            ,
            <given-names>G.F.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Marczewski</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Hochleitner</surname>
            ,
            <given-names>W.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Lankes</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Tscheligi</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          :
          <source>UMAP 2017 Fifty Shades of Personalization - Workshop on Personalization in Serious and Persuasive Games and Gameful Interactions : Organizers ' Welcome. In: Proceedings of UMAP'17 Adjunct</source>
          . pp.
          <fpage>395</fpage>
          -
          <lpage>397</lpage>
          . ACM, Bratislava, Slovakia (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref9">
        <mixed-citation>
          9.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Nacke</surname>
            ,
            <given-names>L.E.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>DiMarco</surname>
          </string-name>
          , C.:
          <article-title>Towards Personality-driven Persuasive Health Games and Gamified Systems</article-title>
          .
          <source>In: Proceedings of the SIGCHI Conference on Human Factors in Computing Systems - CHI '17</source>
          . pp.
          <fpage>1015</fpage>
          -
          <lpage>1027</lpage>
          . ACM (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref10">
        <mixed-citation>
          10.
          <string-name>
            <surname>Ferro</surname>
            ,
            <given-names>L.S.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Walz</surname>
            ,
            <given-names>S.P.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Greuter</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          :
          <article-title>Towards personalised, gamified systems: an investigation into game design, personality and player typologies</article-title>
          .
          <source>In: Proceedings of the 9th Australasian Conference on Interactive Entertainment: Matters of Life and Death - IE '13</source>
          . pp.
          <fpage>1</fpage>
          -
          <lpage>6</lpage>
          (
          <year>2013</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref11">
        <mixed-citation>
          11.
          <string-name>
            <surname>Jia</surname>
            ,
            <given-names>Y.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Xu</surname>
            ,
            <given-names>B.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Karanam</surname>
            ,
            <given-names>Y.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Voida</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          :
          <article-title>Personality-targeted Gamification: A Survey Study on Personality Traits and Motivational Affordances</article-title>
          .
          <source>In: Proceedings of the 34th Annual ACM Conference on Human Factors in Computing Systems - CHI '16</source>
          (
          <year>2016</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref12">
        <mixed-citation>
          12.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Oyibo</surname>
            ,
            <given-names>K.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Tondello</surname>
            ,
            <given-names>G.F.</given-names>
          </string-name>
          :
          <article-title>A Comparison of System-Controlled and UserControlled Personalization Approaches</article-title>
          . In: UMAP 2017 -
          <article-title>Adjunct Publication of the 25th Conference on User Modeling, Adaptation and Personalization (</article-title>
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref13">
        <mixed-citation>
          13.
          <string-name>
            <surname>Tondello</surname>
            ,
            <given-names>G.F.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Wehbe</surname>
            ,
            <given-names>R.R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Diamond</surname>
            ,
            <given-names>L.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Busch</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Marczewski</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Nacke</surname>
            ,
            <given-names>L.E.</given-names>
          </string-name>
          :
          <article-title>The Gamification User Types Hexad Scale</article-title>
          .
          <source>In: Proceedings of the 2016 Annual Symposium on Computer-Human Interaction in Play - CHI PLAY '16</source>
          . pp.
          <fpage>229</fpage>
          -
          <lpage>243</lpage>
          . ACM, Austin, TX, USA (
          <year>2016</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref14">
        <mixed-citation>
          14.
          <string-name>
            <surname>Tondello</surname>
            ,
            <given-names>G.F.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Mora</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Nacke</surname>
            ,
            <given-names>L.E.</given-names>
          </string-name>
          :
          <article-title>Elements of Gameful Design Emerging from User Preferences</article-title>
          .
          <source>In: Proceedings of the 2017 Annual Symposium on Computer-Human Interaction in Play - CHI PLAY '17</source>
          . pp.
          <fpage>129</fpage>
          -
          <lpage>142</lpage>
          . ACM, Amsterdam, Netherlands (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref15">
        <mixed-citation>
          15.
          <string-name>
            <surname>Tondello</surname>
            ,
            <given-names>G.F.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Nacke</surname>
            ,
            <given-names>L.E.</given-names>
          </string-name>
          :
          <article-title>Recommender Systems for Personalized Gamification</article-title>
          .
          <source>In: Proceedings of UMAP'17 Adjunct. ACM</source>
          , Bratislava, Slovakia (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref16">
        <mixed-citation>
          16.
          <string-name>
            <surname>Ricci</surname>
            ,
            <given-names>F.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Rokach</surname>
            ,
            <given-names>L.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Shapira</surname>
            ,
            <given-names>B.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Kantor</surname>
          </string-name>
          , P.B. eds: Recommender Systems Handbook. Springer US (
          <year>2011</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref17">
        <mixed-citation>
          17.
          <string-name>
            <surname>Hamari</surname>
            ,
            <given-names>J.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Tuunanen</surname>
          </string-name>
          , J.:
          <article-title>Player types: A meta-synthesis</article-title>
          .
          <source>Trans. Digit. Games Res</source>
          .
          <volume>1</volume>
          , (
          <year>2014</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref18">
        <mixed-citation>
          18.
          <string-name>
            <surname>Yee</surname>
            ,
            <given-names>N.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Ducheneaut</surname>
            ,
            <given-names>N.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Nelson</surname>
            ,
            <given-names>L.</given-names>
          </string-name>
          :
          <article-title>Online Gaming Motivations Scale: Development and Validation</article-title>
          .
          <source>In: Proceedings of the SIGCHI Conference on Human Factors in Computing Systems</source>
          . pp.
          <fpage>2803</fpage>
          -
          <lpage>2806</lpage>
          (
          <year>2012</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref19">
        <mixed-citation>
          19.
          <string-name>
            <surname>Peever</surname>
            ,
            <given-names>N.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Johnson</surname>
            ,
            <given-names>D.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Gardner</surname>
          </string-name>
          , J.: Personality &amp;
          <article-title>Video Game Genre Preferences</article-title>
          .
          <source>In: Proceedings of The 8th Australasian Conference on Interactive Entertainment: Playing the System</source>
          . p.
          <volume>20</volume>
          (
          <year>2012</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref20">
        <mixed-citation>
          20.
          <string-name>
            <surname>Nacke</surname>
            ,
            <given-names>L.E.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Bateman</surname>
            ,
            <given-names>C.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Mandryk</surname>
            ,
            <given-names>R.L.</given-names>
          </string-name>
          :
          <article-title>BrainHex: A Neurobiological Gamer Typology Survey</article-title>
          .
          <source>Entertain. Comput. 5</source>
          ,
          <fpage>55</fpage>
          -
          <lpage>62</lpage>
          (
          <year>2014</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref21">
        <mixed-citation>
          21.
          <string-name>
            <surname>Bartle</surname>
          </string-name>
          , R.: Hearts, Clubs, Diamonds, Spades:
          <article-title>Players who suit MUDs</article-title>
          .
          <source>J. MUD Res</source>
          .
          <volume>1</volume>
          , (
          <year>1996</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref22">
        <mixed-citation>
          22.
          <string-name>
            <surname>Yee</surname>
          </string-name>
          , N.:
          <article-title>Gamer Motivation Model Overview</article-title>
          and Descriptions, Online: http://quanticfoundry.com/
          <year>2015</year>
          /12/15/handy-reference/.
        </mixed-citation>
      </ref>
      <ref id="ref23">
        <mixed-citation>
          23.
          <string-name>
            <surname>Marczewski</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          :
          <article-title>User Types</article-title>
          . In: Even Ninja Monkeys Like to Play: Gamification,
          <string-name>
            <given-names>Game</given-names>
            <surname>Thinking</surname>
          </string-name>
          &amp; Motivational Design. pp.
          <fpage>69</fpage>
          -
          <lpage>84</lpage>
          . CreateSpace Independent Publishing Platform (
          <year>2015</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref24">
        <mixed-citation>
          24.
          <string-name>
            <surname>Marczewski</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          :
          <article-title>Gamification Mechanics and Elements</article-title>
          . In: Even Ninja Monkeys Like to Play: Gamification,
          <string-name>
            <given-names>Game</given-names>
            <surname>Thinking</surname>
          </string-name>
          &amp; Motivational Design. pp.
          <fpage>165</fpage>
          -
          <lpage>177</lpage>
          . CreateSpace Independent Publishing Platform (
          <year>2015</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref25">
        <mixed-citation>
          25.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Tondello</surname>
            ,
            <given-names>G.F.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Nacke</surname>
            ,
            <given-names>L.E.</given-names>
          </string-name>
          :
          <article-title>Personalizing Persuasive Strategies in Gameful Systems to Gamification User Types</article-title>
          .
          <source>In: Proceedings of the SIGCHI Conference on Human Factors in Computing Systems - CHI '18</source>
          .
          <string-name>
            <surname>ACM</surname>
          </string-name>
          (
          <year>2018</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref26">
        <mixed-citation>
          26.
          <string-name>
            <surname>Goldberg</surname>
            ,
            <given-names>L.R.:</given-names>
          </string-name>
          <article-title>The structure of phenotypic personality traits</article-title>
          .
          <source>Am. Psychol</source>
          .
          <volume>48</volume>
          ,
          <issue>26</issue>
          (
          <year>1993</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref27">
        <mixed-citation>
          27.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Busch</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Dijkstra</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Reisinger</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Stibe</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Tscheligi</surname>
            ,
            <given-names>M.:</given-names>
          </string-name>
          <article-title>Personalization in Persuasive Technology</article-title>
          .
          <source>In: Adjunct Proceedings of the 11th International Conference on Persuasive Technology</source>
          . pp.
          <fpage>96</fpage>
          -
          <lpage>99</lpage>
          (
          <year>2016</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref28">
        <mixed-citation>
          28.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Moffatt</surname>
            ,
            <given-names>K.</given-names>
          </string-name>
          :
          <article-title>Persuasive technology for health and wellness: State-of-the-art and emerging trends</article-title>
          .
          <source>Health Informatics J</source>
          . (
          <year>2016</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref29">
        <mixed-citation>
          29.
          <string-name>
            <surname>Alkış</surname>
            ,
            <given-names>N.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Taşkaya Temizel</surname>
            ,
            <given-names>T.</given-names>
          </string-name>
          :
          <article-title>The impact of individual differences on influence strategies</article-title>
          .
          <source>Pers. Individ. Dif</source>
          .
          <volume>87</volume>
          ,
          <fpage>147</fpage>
          -
          <lpage>152</lpage>
          (
          <year>2015</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref30">
        <mixed-citation>
          30.
          <string-name>
            <surname>Arteaga</surname>
            ,
            <given-names>S.M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Kudeki</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Woodworth</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Kurniawan</surname>
            ,
            <given-names>S.:</given-names>
          </string-name>
          <article-title>Mobile system to motivate teenagers' physical activity</article-title>
          .
          <source>In: Proceedings of the 9th International Conference on Interaction Design and Children - IDC '10</source>
          . pp.
          <fpage>1</fpage>
          -
          <lpage>10</lpage>
          . ACM (
          <year>2010</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref31">
        <mixed-citation>
          31.
          <string-name>
            <surname>Halko</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Kientz</surname>
            ,
            <given-names>J.A.</given-names>
          </string-name>
          :
          <article-title>Personality and Persuasive Technology: An Exploratory Study on Health-Promoting Mobile Applications</article-title>
          . In: Persuasive Technology - PERSUASIVE
          <year>2010</year>
          . pp.
          <fpage>150</fpage>
          -
          <lpage>161</lpage>
          . Springer, Berlin, Heidelberg (
          <year>2010</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref32">
        <mixed-citation>
          32.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Mandryk</surname>
            ,
            <given-names>R.L.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Vassileva</surname>
          </string-name>
          , J.: Gender, Age, and
          <article-title>Responsiveness to Cialdini's Persuasion Strategies</article-title>
          .
          <source>In: International Conference on Persuasive Technology (PERSUASIVE</source>
          <year>2015</year>
          ). pp.
          <fpage>147</fpage>
          -
          <lpage>159</lpage>
          . Springer (
          <year>2015</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref33">
        <mixed-citation>
          33.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Mandryk</surname>
            ,
            <given-names>R.L.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Vassileva</surname>
          </string-name>
          , J.:
          <article-title>Gender and Persuasive Technology: Examining the Persuasiveness of Persuasive Strategies by Gender Groups</article-title>
          .
          <source>In: International Conference on Persuasive Technology (PERSUASIVE</source>
          <year>2014</year>
          ). Springer (
          <year>2014</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref34">
        <mixed-citation>
          34.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Mandryk</surname>
            ,
            <given-names>R.L.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Vassileva</surname>
            ,
            <given-names>J.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Gerling</surname>
            ,
            <given-names>K.M.:</given-names>
          </string-name>
          <article-title>Tailoring persuasive health games to gamer type</article-title>
          .
          <source>In: Proceedings of the SIGCHI Conference on Human Factors in Computing Systems - CHI '13</source>
          . pp.
          <fpage>2467</fpage>
          -
          <lpage>2476</lpage>
          (
          <year>2013</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref35">
        <mixed-citation>
          35.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Vassileva</surname>
            ,
            <given-names>J.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Mandryk</surname>
            ,
            <given-names>R.L.</given-names>
          </string-name>
          :
          <article-title>Modeling the efficacy of persuasive strategies for different gamer types in serious games for health. User Model. User-adapt</article-title>
          .
          <source>Interact</source>
          .
          <volume>24</volume>
          ,
          <fpage>453</fpage>
          -
          <lpage>498</lpage>
          (
          <year>2014</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref36">
        <mixed-citation>
          36.
          <string-name>
            <surname>Khaled</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Barr</surname>
            ,
            <given-names>P.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Noble</surname>
            ,
            <given-names>J.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Fischer</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Biddle</surname>
          </string-name>
          , R.:
          <article-title>Our Place or Mine? Exploration into Collectivism-Focused Persuasive Technology Design</article-title>
          . In: Persuasive Technology - PERSUASIVE
          <year>2016</year>
          . pp.
          <fpage>72</fpage>
          -
          <lpage>83</lpage>
          . Springer, Berlin, Heidelberg (
          <year>2006</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref37">
        <mixed-citation>
          37.
          <string-name>
            <surname>Orji</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Mandryk</surname>
            ,
            <given-names>R.L.</given-names>
          </string-name>
          :
          <article-title>Developing culturally relevant design guidelines for encouraging healthy eating behavior</article-title>
          .
          <source>Int. J. Hum. Comput. Stud</source>
          .
          <volume>72</volume>
          ,
          <fpage>207</fpage>
          -
          <lpage>223</lpage>
          (
          <year>2014</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref38">
        <mixed-citation>
          38.
          <string-name>
            <surname>Kaptein</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Markopoulos</surname>
            ,
            <given-names>P.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>De Ruyter</surname>
            ,
            <given-names>B.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Aarts</surname>
          </string-name>
          , E.:
          <article-title>Personalizing persuasive technologies: Explicit and implicit personalization using persuasion profiles</article-title>
          .
          <source>Int. J. Hum. Comput. Stud</source>
          .
          <volume>77</volume>
          ,
          <fpage>38</fpage>
          -
          <lpage>51</lpage>
          (
          <year>2015</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref39">
        <mixed-citation>
          39.
          <string-name>
            <surname>Kaptein</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>De Ruyter</surname>
            ,
            <given-names>B.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Markopoulos</surname>
            ,
            <given-names>P.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Aarts</surname>
            ,
            <given-names>E.</given-names>
          </string-name>
          :
          <article-title>Adaptive Persuasive Systems: A Study of Tailored Persuasive Text Messages to Reduce Snacking</article-title>
          .
          <source>ACM Trans. Interact. Intell. Syst. 2</source>
          ,
          <fpage>1</fpage>
          -
          <lpage>25</lpage>
          (
          <year>2012</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref40">
        <mixed-citation>
          40.
          <string-name>
            <surname>Mora</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Riera</surname>
            ,
            <given-names>D.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>González</surname>
            ,
            <given-names>C.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Arnedo-Moreno</surname>
            ,
            <given-names>J.</given-names>
          </string-name>
          :
          <article-title>Gamification: a systematic review of design frameworks</article-title>
          .
          <source>J. Comput. High. Educ</source>
          .
          <volume>1</volume>
          -
          <fpage>33</fpage>
          (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref41">
        <mixed-citation>
          41.
          <string-name>
            <surname>Morschheuser</surname>
            ,
            <given-names>B.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Werder</surname>
            ,
            <given-names>K.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Hamari</surname>
            ,
            <given-names>J.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Abe</surname>
          </string-name>
          , J.:
          <article-title>How to gamify? A method for designing gamification</article-title>
          .
          <source>In: Proceedings of the 50th Annual Hawaii International Conference on System Sciences (HICSS)</source>
          . IEEE, Hawaii, USA (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref42">
        <mixed-citation>
          42.
          <string-name>
            <surname>Ferro</surname>
            ,
            <given-names>L.S.</given-names>
          </string-name>
          : Projekt.ID:
          <article-title>Investigating how game elements and mechanics can be aligned to players to preferences</article-title>
          ,
          <source>PhD Thesis</source>
          , RMIT University (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref43">
        <mixed-citation>
          43.
          <string-name>
            <surname>Khoshkangini</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Valetto</surname>
            ,
            <given-names>G.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Marconi</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          :
          <article-title>Generating personalized challenges to enhance the persuasive power of gamification</article-title>
          .
          <source>In: Proceedings of the Second International Workshop on Personalization in Persuasive Technology</source>
          . pp.
          <fpage>70</fpage>
          -
          <lpage>83</lpage>
          . CEUR-WS.org (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref44">
        <mixed-citation>
          44.
          <string-name>
            <surname>Altmeyer</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Lessel</surname>
            ,
            <given-names>P.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Krüger</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          :
          <article-title>Expense Control: A Gamified, Semi-</article-title>
          <string-name>
            <surname>Automated</surname>
          </string-name>
          ,
          <article-title>Crowd-Based Approach For Receipt Capturing</article-title>
          .
          <source>In: Proceedings of the 21st International Conference on Intelligent User Interfaces - IUI '16</source>
          . pp.
          <fpage>31</fpage>
          -
          <lpage>42</lpage>
          . ACM (
          <year>2016</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref45">
        <mixed-citation>
          45.
          <string-name>
            <surname>Lessel</surname>
            ,
            <given-names>P.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Altmeyer</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Müller</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Wolff</surname>
            ,
            <given-names>C.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Krüger</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          :
          <article-title>Measuring the effect of “bottom-up” gamification in a microtask setting</article-title>
          .
          <source>In: Proceedings of the 21st International Academic Mindtrek Conference - AcademicMindtrek '17</source>
          . pp.
          <fpage>63</fpage>
          -
          <lpage>72</lpage>
          . ACM (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref46">
        <mixed-citation>
          46.
          <string-name>
            <surname>Landers</surname>
            ,
            <given-names>R.N.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Bauer</surname>
            ,
            <given-names>K.N.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Callan</surname>
            ,
            <given-names>R.C.</given-names>
          </string-name>
          :
          <article-title>Gamification of task performance with leaderboards: A goal setting experiment</article-title>
          .
          <source>Comput. Human Behav</source>
          .
          <volume>71</volume>
          ,
          <fpage>508</fpage>
          -
          <lpage>515</lpage>
          (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
    </ref-list>
  </back>
</article>