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  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Adaptive Learning Capability: User-Centered Learning at the Next Level</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Barbara Buck</string-name>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Ph.D.</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Matt Genova</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Brandt Dargue</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Elizabeth Biddle</string-name>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Ph.D.</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>The Boeing Company</institution>
          ,
          <addr-line>Orlando FL 32826</addr-line>
          ,
          <country country="US">USA</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>The Boeing Company</institution>
          ,
          <addr-line>St. Louis MO 63166</addr-line>
          ,
          <country country="US">USA</country>
        </aff>
      </contrib-group>
      <fpage>3</fpage>
      <lpage>10</lpage>
      <abstract>
        <p>For more than a decade, Boeing has had an ongoing program of research focused on user-centered adaptive learning. These efforts have been concentrated on the development of two different flavors of adaptive learning. Our Intelligent Tutoring System (ITS) provides a rich personalized student-centered learning experience through the modeling of system knowledge, problem-solving rules, and real-time assessment of student performance. The learning experience provides dynamic scenario sequencing, tailored student feedback and student performance summary based on the perceived student strengths and weaknesses. In the second implementation, we have extended the adaptive learning capability to simulation-based instruction with the Virtual Instructor (VI). The VI provides adaptive simulation- or game-based instruction by monitoring student actions and simulation events, evaluating student performance in real time for a complex set of behaviors, providing information, hints, learning feedback and recommendations to the student and/or instructor. In this paper, we will discuss two specific prototypes of adaptive learning leveraging those implementations. In the first, we have been working with the U.S. Army Research Laboratory (ARL) to integrate our adaptive learning capability with the ARL's Generalized Intelligent Framework for Tutoring (GIFT). The product is an integrated adaptive prototype that we plan to evaluate as part of an effectiveness study this coming year. In the second implementation, we are developing an intelligent virtual reality-based teammate to enable training of individual technical and teamwork tasks within an intelligent tutoring environment. This synthetic teammate will be integrated with the VI capability to respond to the student in real time to support team training objectives. We will discuss the successes and challenges encountered as we have developed these prototype capabilities.</p>
      </abstract>
      <kwd-group>
        <kwd>Intelligent Tutoring</kwd>
        <kwd>Adaptive Learning</kwd>
        <kwd>Performance Assessment</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>-</title>
      <p>
        Each person is unique. People come from different backgrounds, different beliefs,
different experiences and they have different goals. As a result, people learn at different
rates in different ways. Adaptivity is the ability of a system to alter (change) itself to
better fit or function in a given situation. In order to optimize the learning experience
for a unique person, a learning system should adapt to the individual learner or team
for the specific situation. The notion that instruction should adapt to the learner is not
new. Effective instructors and mentors have adapted to a specific learner’s needs since
humans started teaching other humans. The goal of an Intelligent Tutoring System
(ITS) then is to provide automated instruction equivalent to that of a skilled human
tutor. Automating instructional adaptivity is also not new, but has been somewhat
elusive and various techniques have been tried. As early as 1958, the famed psychologist,
B.F. Skinner experimented with Artificial Intelligence and Behaviorism. ITS
development has gained momentum since the 1980’s, with numerous automated tutors being
developed and applied in both university and Department of Defense settings [
        <xref ref-type="bibr" rid="ref1 ref2 ref3 ref4 ref5 ref6">1, 2, 3,
4, 5, 6</xref>
        ]. While there is much to learn in this area, many approaches have been
successful. A meta-analysis [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ] of 50 controlled experiments showed:
• Students who received intelligent tutoring outperformed students from conventional
classes in 92% of the controlled evaluations.
• Improvement in performance was substantive in 78% of the controlled evaluations.
• The median effect size was considered moderate-to-large effect for studies in the
social sciences.
      </p>
      <p>
        Developing expertise is time-consuming and difficult. So how do we optimize a
person’s performance to most efficiently develop expertise? In his book, Flow: The
Psychology of Optimal Experience, Csíkszentmihályi [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ] described the state of flow as
the ultimate experience in learning and performing. World-class experts describe flow
as a state of hyper-efficiency in performing a task, as if there was a current of water
carrying them along. Flow theory postulates three conditions that must be met to
achieve a flow state:
• One must be involved in an activity with a clear set of goals and progress.
• The task at hand must have clear and immediate feedback.
• One must have a good balance between the perceived challenges of the task at hand
and his or her own perceived skills.
      </p>
      <p>
        Thus, our approach to ITS development has been on building tools and techniques
to place and keep students in the flow of optimal learning. Studies by Boeing [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ] and
others [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ] show that adaptive training that provides structured practice and assessment
with feedback can provide highly effective results. This paper describes our approach
to adaptive training development, and describes experience we have had in creating
adaptive learning prototypes.
2
      </p>
    </sec>
    <sec id="sec-2">
      <title>Adaptive Training Development Experience</title>
      <p>Boeing’s approach to a learner-centered adaptive training implementation has evolved
over the course of the past few years. Initial implementations focused on creation of
an architecture and authoring solution in support of intelligent tutoring. The product of
this effort was Web-based, SCORM®-conformant computer-based training. More
recent efforts focused on development of simulation-based instruction that monitors
student performance in real time and adapts the scenario accordingly. More details of both
approaches are provided below.
2.1</p>
      <sec id="sec-2-1">
        <title>ITS Implementation</title>
        <p>Figure 1 is an overview of our implementation of the ITS. The design features 3
components: a Student Model, an Instructional Model, and an Expert Model. The student
model implements a profile of dynamically maintained variables, each corresponding
to one learning objective. These variables are evaluated over a number of observations.
As a result, changes due to learning are reflected across exercises, as the score increases
due to correct performance, or decreases as errors are made. The amount that scores
are changed can be weighted according to the degree to which the action reflects
mastery of the learning objective. Amount of change is also adjusted according to the
degree of support provided to the student by the ITS in selecting this action.</p>
        <p>The instructional model responds to student requests for help or student errors with
information on problem-solving strategies. The specificity of the information increases
as additional requests are made or additional errors occur. The instructional model is
also tasked with providing within-scenario feedback to guide the student, as well as
performance summaries across all learning objectives at the end of the lesson scenario.</p>
        <p>
          Our implementation of the expert model is based on a cognitive task analysis
technique known as PARI, for Precursor, Action, Results, and Interpretation [
          <xref ref-type="bibr" rid="ref11">11</xref>
          ]. PARI
provides methods to elicit detailed information from experts on how they represent a
given state of a solution (what issues have been resolved and what issues remain),
optimal and alternative paths to a solution, and their strategies for selecting actions at each
step along those paths. The expert model directly encodes these solution paths. For
each path, the model also encodes the expert’s summary of the situation (representation
of the problem) and the rationales for the possible next steps. We have published details
of the ITS architecture and implementation elsewhere [
          <xref ref-type="bibr" rid="ref12">12</xref>
          ].
2.2
        </p>
      </sec>
      <sec id="sec-2-2">
        <title>VI Implementation</title>
        <p>Extending the ITS adaptive training approach to the more dynamic simulation-based
training environment is the goal of the VI implementation. Whether it be a desktop or
networked simulation, or even a game-based simulation environment, there are a
number of challenges related to the real-time assessment of student performance and
scenario adaptation within this fast-paced environment. Similar to our ITS approach, our
basic architecture involves a student, expert and instructional model.</p>
        <p>Inferring student intent can be more complex in these dynamic environments where
even a large variance from the expert over a period of time is a reasonable alternative
and not necessarily a “mistake”. The VI implementations use interpretation based on
multiple student actions within the scenario context. Contrasted to the ITS, where a
single response to a question was the norm, in the VI students may complete any
number of actions. Often times, multiple action sequences are equally correct. Our
approach to student modeling utilized behavior trees, where an action is interpreted within
the context of a given branching structure. A tree can be activated as behaviors are
recognized, and multiple trees may be active in parallel. Performance is assessed
against detailed learning objectives and feedback is provided based on the interpretation
of performance and in a format that is compatible with the particular simulation.
Networking capabilities are employed to report performance to any number of data logging
or learning records capabilities.</p>
        <p>The VI is set up to run as an independent instructional tool to assess performance
and provide student feedback in the absence of a human instructor, or may be employed
to enhance instructor-based learning through objective metrics tracking, real-time
notifications to the instructor and enhanced after-action review.
3
3.1</p>
      </sec>
    </sec>
    <sec id="sec-3">
      <title>Prototype Development</title>
      <sec id="sec-3-1">
        <title>ITS/GIFT Prototype</title>
        <p>
          The Generalized Intelligent Framework for Tutoring (GIFT) program is a U.S. Army
Research Laboratory (ARL) effort to develop a framework for personalized,
on-demand, computer based instruction to improve the speed and quality of Soldier training
[
          <xref ref-type="bibr" rid="ref13">13</xref>
          ]. As part of a three-year cooperative research and development agreement, we have
been working with ARL to develop an integrated adaptive prototype in which we
combine the Army’s GIFT adaptive learning framework with our ITS and VI capabilities.
        </p>
        <p>
          The prototype uses an aircraft maintenance scenario with aspects of troubleshooting
and part replacement. It uses the knowledge assessment functionality and individual
difference categorization within GIFT to sequence course content to the student and to
adapt course content based on ongoing student parameter characterization. Our ITS
capability provides lesson content for required learning, and adapts within-lesson
content to maximize a student’s ability to successfully pass lesson modules on the initial
attempt. The final evaluated practice module is completed using our virtual
maintenance capability known as Advanced Deployable Accelerated Personalized Training
(ADAPT) [
          <xref ref-type="bibr" rid="ref14">14</xref>
          ]. As part of the final practice assessment, students don a virtual reality
(VR) headset, and using two 3D VR hand controllers, they are able to navigate to
various places on the aircraft, perform the required troubleshooting tasks while adhering to
required safety protocols, diagnose the fault and replace the faulty part (Figure 2). The
VI within the ADAPT system scores the student on targeted learning objectives,
provides on-demand student assistance to help locate components, and provides scoring to
determine whether the student passes or fails the practical assessment.
        </p>
        <p>The first iteration of the integrated prototype has been completed. Current efforts
are focused on development of a test plan for the conduct of an adaptive training
effectiveness study. Once the design is complete, any required modifications will be made
to the adaptive training prototype in support of the effectiveness study and we will
collect data to evaluate which adaptive training implementations resulted in reduced
time to competence, improved performance outcomes, more effective training transfer
and knowledge retention. Findings from the effectiveness study will be used to modify
the framework for GIFT, as well as our adaptive training approach.</p>
      </sec>
      <sec id="sec-3-2">
        <title>Virtual Pilot Development</title>
        <p>Training to develop flight crew coordination skill is gaining focus in the commercial
aviation community. Currently, flight crew coordination is embedded within flight
scenarios performed in large full flight simulators (FFS). Additionally, there are times
when an airline is not able to send two pilots to train together, requiring an instructor to
play the role of the other pilot. Consequently, there are limited opportunities to hone
these competencies. To address this need, we are developing a Virtual Pilot to enable
student pilots to conduct flight crew coordination training on their own, without
needing a second pilot, instructor or even the use of a FFS.</p>
        <p>The Virtual Pilot can be used in an Augmented Reality (AR) or VR environment.
The AR use case is to support crew coordination training when the student is in a
traditional training device such as a FFS or flat panel trainer, but another pilot is not
available to train. In this case the student interacts with the Virtual Pilot wearing AR
goggles. In VR mode, the Virtual Pilot is integrated with a VR flight deck environment
and the student interacts using a VR headset. In both cases, the student’s speech, inputs
to the flight deck, and movements are used by the Virtual Pilot to determine how to
respond. To support cases in which an instructor is not available, the Virtual Pilot is
integrated with a version of the VI called the VR Instructor that will guide training,
monitor progress, provide feedback and interject events or scenarios into training for
the purpose of challenging the student or addressing an identified training need.</p>
        <p>When integrated with the VI, the Virtual Pilot is capable of performing assigned
flight tasks (e.g., role of the Captain) and interact with the student pilot through speech
and gestures. The VI will receive the same data inputs from the student as the Virtual
Pilot – speech, flight deck interaction and gestures/head movement – and use this data
to evaluate the student’s performance against pre-defined performance measures, as
described previously. The VI may provide feedback in terms of verbal or textual
comments, or by highlighting areas in the cockpit visually, or even providing a vibration or
other tactile indicator, such as in the case of directing the student’s attention to a
particular instrument. Additionally, the VI may command the Virtual Pilot to perform a
task incorrectly depending upon the teaching point to be made. For example, if the
student appears to not be monitoring and responding to Virtual Pilot’s actions, the VI
may command the Virtual Pilot to perform a task incorrectly for the purpose of
prompting the student pilot to speak up and intervene.
4</p>
      </sec>
    </sec>
    <sec id="sec-4">
      <title>Challenges and Future Directions</title>
      <p>This paper describes some unique approaches to creating adaptive training solutions.
While focused on different applications, both solutions attempt to employ the same
basic underlying concepts to develop expertise based on optimizing a learning
experience by adapting to the student. One takeaway is that with multiple approaches to
adaptivity, each method has challenges. We have learned through experience that there
are strengths and weaknesses of different approaches to modeling students, providing
feedback, and adapting content. By integrating the Boeing adaptive learning
approaches with GIFT, we have identified certain communication and software
compatibility challenges. These will be mitigated as we continue to work on a mutual joint
solution.</p>
      <p>Adaptive learning within simulated training environments can be challenging if
access to information for assessing performance have not been built into the simulator or
gaming engine. Many times, simulators communicate performance at the mission level,
whereas student performance needs to be evaluated at the switch or button push level.
Adding the capability to perform automated performance assessment within the
simulation proves costly and time-consuming. However, adding access to the events and
data at the switch or button level for evaluation by tools like the ITS and VI have proven
to be minimal.</p>
      <p>Finally, adaptive training has yet to be widely accepted within the educational
community. We speculate that this is due in part to the added cost of creating multiple
sources of adaptive content (something that is getting better with continuous
improvements in authoring capability), as well as a potential increased time to proficiency based
on student performance. While ample evidence documents improvements to student
training performance, training transfer and long-term knowledge retention based on
adaptive training solutions, there is reluctance to adopt these approaches given the
potential added complications of each student progressing at his or her own pace.</p>
      <p>Near-term future plans include the conduct of studies to evaluate the effectiveness
our own adaptive training approaches. Plans are in work to evaluate the effectiveness
of the Boeing/GIFT prototype. We will be using cadets at West Point to assess various
manipulations of overall curriculum adaptation in an effort to determine which are best
utilized to optimize student performance. Based on the results of this study, we plan to
modify the prototype to better meet the needs of students. Work continues on the
development of the Virtual Pilot to integrate the adaptive lesson content and feedback
with its physical avatar. A study is planned to validate the effectiveness of the virtual
adaptive learning with this implementation as well.</p>
    </sec>
  </body>
  <back>
    <ref-list>
      <ref id="ref1">
        <mixed-citation>
          1.
          <string-name>
            <surname>Bloom</surname>
            ,
            <given-names>B. S.</given-names>
          </string-name>
          <article-title>The 2 sigma problem: The search for methods of group instruction as effective as one-to-one tutoring</article-title>
          .
          <source>Educational Researcher</source>
          ,
          <volume>13</volume>
          , pp.
          <fpage>4</fpage>
          -
          <lpage>16</lpage>
          . (
          <year>1984</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref2">
        <mixed-citation>
          2.
          <string-name>
            <surname>Lesgold</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Lajoie</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Bunzo</surname>
            ,
            <given-names>M.</given-names>
          </string-name>
          , &amp;
          <string-name>
            <surname>Eggan</surname>
            ,
            <given-names>G.</given-names>
          </string-name>
          <article-title>SHERLOCK: A coached practice environment for an electronics troubleshooting job</article-title>
          . Pittsburgh: University of Pittsburgh, Learning Research and Development Center. (
          <year>1988</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref3">
        <mixed-citation>
          3.
          <string-name>
            <surname>Anderson</surname>
            ,
            <given-names>J.R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Corbett</surname>
            ,
            <given-names>A.T.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Koedinger</surname>
            ,
            <given-names>K.R.</given-names>
          </string-name>
          , and
          <string-name>
            <surname>Pelletier</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          <string-name>
            <surname>Cognitive</surname>
          </string-name>
          <article-title>Tutors: Lessons Learned</article-title>
          .
          <source>The Journal of the Learning Sciences. 4</source>
          (
          <issue>2</issue>
          ):
          <fpage>167</fpage>
          -
          <lpage>207</lpage>
          . (
          <year>1995</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref4">
        <mixed-citation>
          4.
          <string-name>
            <surname>Hunt</surname>
            ,
            <given-names>E.</given-names>
          </string-name>
          and
          <string-name>
            <surname>Minstrell</surname>
            ,
            <given-names>J.</given-names>
          </string-name>
          (
          <year>1994</year>
          ).
          <article-title>A collaborative classroom for teaching conceptual physics</article-title>
          . In K. McGilly (Ed.),
          <article-title>Classroom lessons: Integrating cognitive theory and classroom practice</article-title>
          . Cambridge: MIT Press
        </mixed-citation>
      </ref>
      <ref id="ref5">
        <mixed-citation>
          5.
          <string-name>
            <surname>Graesser</surname>
            ,
            <given-names>A. C.</given-names>
          </string-name>
          ;
          <string-name>
            <surname>Person</surname>
            ,
            <given-names>N. K.</given-names>
          </string-name>
          <string-name>
            <surname>Question Asking During Tutoring</surname>
          </string-name>
          .
          <source>American Educational Research Journal</source>
          <year>1994</year>
          ,
          <volume>31</volume>
          ,
          <fpage>104</fpage>
          -
          <lpage>137</lpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref6">
        <mixed-citation>
          6.
          <string-name>
            <surname>Cohen</surname>
            ,
            <given-names>P.A.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Kulik</surname>
            ,
            <given-names>J.A.</given-names>
          </string-name>
          and
          <string-name>
            <surname>Kulik</surname>
          </string-name>
          ,
          <string-name>
            <surname>C-L. Educational</surname>
          </string-name>
          <article-title>Outcomes of Tutoring: A Metaanalysis of Findings</article-title>
          . American Educational Research Journal.
          <volume>19</volume>
          :
          <fpage>237</fpage>
          . (
          <year>1982</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref7">
        <mixed-citation>
          7.
          <string-name>
            <surname>Kulik</surname>
            ,
            <given-names>J. A.</given-names>
          </string-name>
          , &amp;
          <string-name>
            <surname>Fletcher</surname>
            ,
            <given-names>J. D.</given-names>
          </string-name>
          <article-title>Effectiveness of Intelligent Tutoring Systems</article-title>
          .
          <source>Review of Educational Research</source>
          ,
          <volume>86</volume>
          (
          <issue>1</issue>
          ),
          <fpage>42</fpage>
          -
          <lpage>78</lpage>
          . doi:
          <volume>10</volume>
          .3102/0034654315581420. (
          <year>2016</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref8">
        <mixed-citation>
          8.
          <string-name>
            <surname>Csíkszentmihályi</surname>
          </string-name>
          , Mihály.
          <source>Flow: The Psychology of Optimal Experience, New York: Harper and Row, ISBN 0-06-092043-2</source>
          . (
          <year>1990</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref9">
        <mixed-citation>
          9.
          <string-name>
            <surname>Perrin</surname>
            ,
            <given-names>B. M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Banks</surname>
            ,
            <given-names>F.</given-names>
          </string-name>
          , &amp;
          <string-name>
            <surname>Dargue</surname>
            ,
            <given-names>B. W.</given-names>
          </string-name>
          <article-title>Student vs. software pacing of instruction: An empirical comparison of effectiveness</article-title>
          .
          <source>The Proceedings of the 2004 Interservice/Industry Training, Simulation, and Education Conference</source>
          . Orlando, FL. (
          <year>2004</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref10">
        <mixed-citation>
          10.
          <string-name>
            <surname>VanLehn</surname>
            ,
            <given-names>K.</given-names>
          </string-name>
          <article-title>The relative effectiveness of human tutoring, intelligent tutoring systems and other tutoring systems</article-title>
          .
          <source>Educational Psychologist</source>
          ,
          <volume>46</volume>
          :4, pp.
          <fpage>197</fpage>
          -
          <lpage>221</lpage>
          . doi:
          <volume>10</volume>
          .1080/00461520.
          <year>2011</year>
          .
          <volume>611369</volume>
          . (
          <year>2011</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref11">
        <mixed-citation>
          11.
          <string-name>
            <surname>Hall</surname>
            ,
            <given-names>E.M.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Gott</surname>
            ,
            <given-names>S.P.</given-names>
          </string-name>
          , &amp;
          <string-name>
            <surname>Pokorny</surname>
            ,
            <given-names>R.A.</given-names>
          </string-name>
          <article-title>A procedural guide to cognitive task analysis: The PARI methodology</article-title>
          .
          <source>Technical Report No. AL/HR-TR-1955-0108</source>
          .
          <string-name>
            <surname>Brooks</surname>
            <given-names>AFB</given-names>
          </string-name>
          , TX: AFMC. (
          <year>1995</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref12">
        <mixed-citation>
          12.
          <string-name>
            <surname>Perrin</surname>
            ,
            <given-names>B.</given-names>
          </string-name>
          <string-name>
            <surname>Intelligent Tutoring</surname>
          </string-name>
          <article-title>Systems: Facilitating Learning While Holding to Standard Practice. Paper presented at the International Training</article-title>
          and Education Conference, Brussels, Belgium. (
          <year>2009</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref13">
        <mixed-citation>
          13.
          <string-name>
            <surname>Sottilare</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Brawner</surname>
            ,
            <given-names>K.</given-names>
          </string-name>
          ,
          <string-name>
            <surname>Sinatra</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          &amp;
          <string-name>
            <surname>Johnston</surname>
            ,
            <given-names>J.</given-names>
          </string-name>
          <article-title>An Updated Concept for a Generalized Intelligent Framework for Tutoring (GIFT)</article-title>
          .
          <source>Technical Report No. DOI: 10.13140/RG.2.2.12941.54244</source>
          .
          <string-name>
            <surname>Orlando</surname>
          </string-name>
          , FL: US Army Research Laboratory. (
          <year>2017</year>
          ).
        </mixed-citation>
      </ref>
      <ref id="ref14">
        <mixed-citation>
          14.
          <string-name>
            <surname>Jacquin</surname>
            , G. V-22
            <given-names>Maintenance</given-names>
          </string-name>
          <string-name>
            <surname>System</surname>
          </string-name>
          .
          <source>St. Louis, MO: Boeing Technical White Paper</source>
          . (
          <year>2016</year>
          ).
        </mixed-citation>
      </ref>
    </ref-list>
  </back>
</article>