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<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta>
      <journal-title-group>
        <journal-title>D. Gorur);</journal-title>
      </journal-title-group>
    </journal-meta>
    <article-meta>
      <title-group>
        <article-title>ArguCast: A System for Online Multi-Forecasting with Gradual Argumentation</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Deniz Gorur</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Antonio Rago</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Francesca Toni</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Department of Computing, Imperial College London</institution>
          ,
          <country country="UK">UK</country>
        </aff>
      </contrib-group>
      <pub-date>
        <year>2023</year>
      </pub-date>
      <volume>000</volume>
      <fpage>0</fpage>
      <lpage>0001</lpage>
      <abstract>
        <p>Judgmental forecasting is a form of forecasting which employs (human) users to make predictions about speci ed future events. Judgmental forecasting has been shown to perform better than quantitative methods for forecasting, e.g. when historical data is unavailable or causal reasoning is needed. However, it has a number of limitations, arising from users' irrationality and cognitive biases. To mitigate against these phenomena, we leverage on computational argumentation, a eld which excels in the representation and resolution of con icting knowledge and human-like reasoning, and propose novel ArguCast frameworks (ACFs) and the novel online system ArguCast, integrating ACFs. ACFs and ArguCast accommodate multi-forecasting, by allowing multiple users to debate on multiple forecasting predictions simultaneously, each potentially admitting multiple outcomes. Finally, we propose a novel notion of user rationality in ACFs based on votes on arguments in ACFs, allowing the ltering out of irrational opinions before obtaining group forecasting predictions by means commonly used in judgmental forecasting.</p>
      </abstract>
      <kwd-group>
        <kwd>eol&gt;Bipolar Argumentation</kwd>
        <kwd>Gradual Semantics</kwd>
        <kwd>Judgmental Forecasting</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>
        Judgmental forecasting is a form of forecasting which employs (human) users to make predictions
about speci ed future events [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ]. It is advocated as a valuable alternative to conventional
quantitative methods for forecasting when historical data is unavailable or causal reasoning
is required [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ]. However, judgmental forecasting has a number of limitations, arising from
(human) users’ irrationality and cognitive biases [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ] arising from over-/under-con dence [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ] in
their judgment. To overcome these issues many solutions have been proposed. Researchers
have investigated the best ways of eliciting probabilities from humans [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ], how incentives
and training change users’ forecasting abilities [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ], and the e ect of scoring rules on users [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ].
Another research direction has focused on employing many experts or humans and aggregating
their predictions since it has been found that group judgment usually performs better as the
impact of bias is reduced by cancelling random error [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ]. However, when there are many humans
involved in forecasting, a new problem arises: how to e ectively combine all the predictions
that are made. A further, orthogonal issue with existing systems, e.g. [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], is that any information
provided by users, e.g. their forecasts or reasoning therefor, concern a single event, and thus
must be provided separately for di erent events. It is easy to see that being able to consider
di erent events in one forecasting framework would utilise the information provided by users
much more e ectively.
      </p>
      <p>
        Meanwhile, argumentation constitutes a major component of human intelligence since the
ability to engage in arguments is essential for humans to understand new problems, perform
scienti c reasoning, and express, clarify, and defend their opinions in their daily lives [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ].
Computational argumentation (see [
        <xref ref-type="bibr" rid="ref8 ref9">8, 9</xref>
        ] for overviews) has become an important topic in arti cial
intelligence due to its ability to conjugate representational needs with user-related cognitive
models and computational models for automated reasoning [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ]. These computational models
are formalised as argumentation frameworks. Argumentation involves reasoning with
uncertainty, and resolving con icting information so we posit that it is natural to apply techniques
from argumentation to the area of judgmental forecasting. However, there has been very little
research in the use of argumentation in forecasting technology in the past. We are aware of
only one such approach [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], which restricts users’ provided information to single events and
single outcomes.
      </p>
      <p>
        In order to address the aforementioned issues, and thus make contributions to the eld of
judgmental forecasting, we leverage on computational argumentation. Speci cally, we propose
the novel ArguCast frameworks (ACFs) and a novel online system ArguCast, integrating ACFs.
Like [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], ACFs (and thus ArguCast) allows for groups of users to make forecasts on events
while engaging in argumentative debates supported by votes on arguments exchanged in these
debates, encouraging users to consider and share their reasoning for their forecasts. However,
di erently from the existing approach, ACFs accommodate multiple forecasting predictions
with multiple outcomes from multiple users, allowing for information to be shared across events.
We also propose a novel notion of user rationality, comprising vote rationality and prediction
rationality in ACFs, allowing the ltering out of irrational opinions before obtaining group
forecasting predictions. In doing so, we provide a novel, argumentative method for combining
forecasts, which introduces multi-forecasting on multiple events simultaneously and accounts
for human biases and rationality.
      </p>
      <p>The structure of the paper is as follows. In Section 2, we describe the most relevant existing
approaches to forecasting. In Section 3, we give the necessary background on computational
argumentation, which is used for de ning rationality. In Section 4, we de ne ACFs. In Section
5, we provide an overview of the implementation of ACFs as the ArguCast system. In Section
6, we de ne our notions of user rationality in ACFs and demonstrate how they can be used
to lter out irrational predictions before we aggregate users’ predictions. Finally, Section 7
concludes and considers possible future directions of work.</p>
    </sec>
    <sec id="sec-2">
      <title>2. Related Work</title>
      <p>In this section, we will discuss the most relevant of the existing approaches to forecasting from
the literature.</p>
      <p>
        There are two approaches to combining forecasts: the qualitative approach (e.g. a group
discussion to reach consensus) and the mechanical approach (e.g. a simple or weighted average
of the forecasts). It has been shown that those which are mechanical are more likely to lead to
greater accuracy than those which are qualitative [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ]. Many ways of mechanical combinations
methods have been proposed before such as linear, log-linear, and democratic opinion pools.
The aggregation method that we use in Section 6 is a variation of the log-linear pooling method.
      </p>
      <p>Some of the systems that attempt to improve the forecasting capability of users by incentives,
e.g. via monetising the users’ predictions based on their accuracy, are Hypermind1, Smarkets2,
PredictIt3, and Polymarket4. The idea behind all of these systems is based on prediction markets,
where people bet on the predictions. Smarkets and PredictIt do not have any functionality for
the agents to debate amongst each other, whereas Polymarket and Hypermind only have a
general chat/forum.</p>
      <p>
        Good Judgment Open (GJOpen)5 [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ], Metaculus6, and Infer7 are examples of group judgment
systems. They can all support binary and multiple-answer questions. In addition to these
Metaculus supports numeric interval and date interval questions. They both have a comment
section for users to put forward their reasoning for their forecast. GJOpen and Infer elicit both
reasoning for why the forecast could be correct, and why the forecast could be wrong from
users, as it has been shown that forcing users to think about why they might be wrong makes
them better forecasters (see Appendix A of [
        <xref ref-type="bibr" rid="ref12">12</xref>
        ]). GJOpen also investigates what would happen
if the top forecasters of their tournaments were put on teams called ‘Superforecasters’ [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ], and
they were found to outperform the simple average of the crowd.
      </p>
      <p>
        However, all of the systems we have discussed lack any mechanisms for eliciting, representing
and evaluating the argumentative reasoning which takes place in the debates amongst users. We
are aware of only Irwin et al.[
        <xref ref-type="bibr" rid="ref6">6</xref>
        ] who have formalised an argumentation framework that supports
forecasting. However, for all its strengths, this approach hosts a number of shortcomings, such
as the fact that it can only handle questions with binary answers and does not allow the same
argument to be used for multiple questions, or even in the same question. This could introduce
repetition and sparsity, which could cause confusion in users. In our novel ArguCast frameworks,
we address all of these issues.
      </p>
    </sec>
    <sec id="sec-3">
      <title>3. Background</title>
      <p>
        Our approach uses Quantitative Bipolar Argumentation Frameworks (QBAFs) [
        <xref ref-type="bibr" rid="ref14">14</xref>
        ]. A QBAF is a
tuple X , A, S, where X , A, S is a Bipolar Argumentation Framework (BAF) [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ] and is a
base score function, such that:
• X is a set of arguments;
• A is a binary relation of attack on X , A X X ;
• S is a binary relation of support on X , S X X ; and
• : X [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ] is a total function; (a) is the base score of a X .
1https://predict.hypermind.com
2https://smarkets.com/
3https://www.predictit.org/
4https://polymarket.com/
5https://www.gjopen.com
6https://www.metaculus.com
7https://www.infer-pub.com
In this paper we focus on the Discontinuity-Free QuAD gradual semantics (DF-QuAD) [
        <xref ref-type="bibr" rid="ref16">16</xref>
        ] for
QBAFs. DF-QuAD determines the strength of arguments based on combining their base scores
and the aggregated strength of their attackers and supporters, where, for a X , the attackers of
a are A(a) = {b|(b, a) A} and the supporters of a are S(a) = {b|(b, a) S} . Let the strength
aggregation function be : [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ] [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ] such that, for T = (v1, . . . , vn) [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ] :
if n = 0 : (T ) = 0;
if n = 1 : (T ) = v1;
if n = 2 : (T ) = f (v1, v2);
if n &gt; 2 : (T ) = f ( (v1, . . . , vn 1), vn)
where, for x, y [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ], f (x, y) = x + (1
be de ned as c : [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ] [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ] [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ]
      </p>
      <p>
        x) · y = x + y x · y. Let the combination function
[
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ], where for v0, va, vs [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ]:
c(v0, va, vs) = v0
c(v0, va, vs) = v0 + (1
v0 · |vs
      </p>
      <p>va|
v0) · |vs
va|
if va
if va &lt; vs.</p>
      <p>
        vs;
Then, DF-QuAD computes the strength of arguments by the score function : X [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ] where,
for any a X , (a) = c( (a), ( (A(a))), ( (S(a)))) such that (A(a)) = ( (a1), . . . , (an)),
where (a1, . . . , an) is an arbitrary permutation of the (n 0) attackers in A(a), and (S(a)) =
( (s1), . . . , (sm)), where (s1, . . . , sm) is an arbitrary permutation of the (m 0) supporters
in S(a).
      </p>
    </sec>
    <sec id="sec-4">
      <title>4. ArguCast Frameworks</title>
      <p>We introduce novel ArguCast frameworks, accommodating multi-forecasting, i.e. multiple
forecasting predictions with multiple outcomes from multiple users, supported by argumentative
debates and votes on arguments exchanged in these debates.</p>
      <p>
        De nition 1. An ArguCast framework (ACF) is a tuple X , R, U , V , P such that:
• X = F D is a nite set of arguments where F and D are disjoint; elements of F and D
are referred to, respectively, as forecasting and non-forecasting arguments;
• R = A S D X , where A and S are disjoint relations (i.e. sets of pairs from D X )
of attack and support, respectively;
• U is a nite set of users;
• V : U D { , +} is a (partial) function; V (u, a) is the vote of user u U on
(nonforecasting) argument a D ;
• P : U F [
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ] is a (partial) function; P (u, b) is the forecasting prediction by user
u U on (forecasting) argument b F .
      </p>
      <p>
        Forecasting arguments represent answers to forecasting questions. There may be any number of
forecasting arguments, as the answers may be Yes/No or take any value in a discrete set (thus the
forecasting predictions may have multiple outcomes). If there is a single forecasting question of
interest admitting a binary answer (e.g. ‘Will Miami Heat win the 2022-23 NBA Championship?’)
then we assume that F is a singleton set consisting of an argument for the positive answer to the
question. If there are multiple forecasting questions or a single forecasting question admitting
several alternative answers (e.g. ‘Which team will win the 2022-23 NBA Championship?’) then
F will consist of several forecasting arguments. Non-forecasting arguments can be seen as
the users’ rationales/opinions around the forecasting arguments. Note that, by de nition of A
and S, forecasting arguments can be attacked/supported but they cannot attack/support other
arguments, whereas non-forecasting arguments can attack/support (or be attacked/supported
by) any arguments, including potentially attacking/supporting more than one argument. The
users are forecasters, who can vote (positively or negatively) on non-forecasting arguments
and/or express a numerical prediction (in [
        <xref ref-type="bibr" rid="ref1">0,1</xref>
        ]) for forecasting arguments. The votes indicate
agreement or disagreement with the non-forecasting argument, whereas prediction forecasts
indicate the users’ degree of belief in the forecasting arguments. Note that, as V and P may be
partial, users may refrain from voting and forecasting.
      </p>
      <p>Example 1. A possible ACF for the question ‘Will large language models (LLMs) reach AGI before
2030?’ is F = {f1 =‘LLMs will reach AGI before 2030’}, D = {d1 =‘As the number of parameters
in LLMs have increased, emergent abilities have been observed’, d2 =‘Mere increase of scale of
machine learning models may not be su cient to reach AGI’ , d3 =‘See paper: "Are Emergent
Abilities of Large Language Models a Mirage?"’}, A = {(d2, f1), (d3, d1)}, S = {(d1, f1)}, U =
{u1, u2}, V(u1, d1) = +, V(u1, d2) = , V(u2, d1) = , V(u2, d3) = +, P(u1, f1) = 0.87,
P(u2, f1) = 0.20, as shown in Figure 1.</p>
      <p>
        Finally, note that our ACFs share some features of existing argumentation frameworks, but
are di erent therefrom as follows. Like BAFs [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ], ACFs use two relations of attack/support
but in addition ACFs distinguish two types of arguments (forecasting and non-forecasting
arguments) and include users, users’ votes on non-forecasting arguments and users’ predictions
on forecasting arguments. Like QuAD frameworks [
        <xref ref-type="bibr" rid="ref17">17</xref>
        ], ACFs single out a speci c type of
argument under debate (forecasting arguments in ACFs but answer arguments in [
        <xref ref-type="bibr" rid="ref17">17</xref>
        ]) but
QuAD frameworks also distinguish con/pro arguments and admit a single relation whereas
we distinguish attack/support relations, allowing for arguments to potentially attack some
arguments and support some others. Also, QuAD frameworks lack users, votes and predictions,
but include base scores for arguments, absent in ACFs (where, however, they can be obtained
using votes, see below). QuaD-V [
        <xref ref-type="bibr" rid="ref18">18</xref>
        ] is an extension of QuAD frameworks like ACFs including
users and votes and excluding base scores. While the votes in QuAD-V are given by a total
function into { , ?, +}, we use a partial function into { , +}. QuAD-V frameworks also lack
support for forecasting predictions. Like the forecasting argumentation frameworks (FAFs) of [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ],
ACFs are designed to support forecasting but FAFs can only handle questions with binary
answers (as they can only have one proposal argument at a time). Like FAFs, ACFs single
out a speci c type of argument under debate (forecasting arguments in ACFs but proposal
arguments in [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]), and they also support users with votes and forecasts. The votes in FAFs are
assigned by a total function that forces users to provide their opinion on every argument. FAFs
also use amendment arguments (arguments proposing the forecasted probability is increased
or decreased) as well as pro/con arguments as in QuAD and V-QuAD frameworks, and a
single relation between arguments, where amendment arguments can only relate to proposal
arguments and con/pro arguments can only relate to amendment and other con/pro arguments.
A further di erence between ACFs and FAFs lies in the fact that, like in QuAD and QuAD-V, FAFs
distinguish con/pro arguments and admit a single relation, which could introduce repetition and
sparsity, which will lead to confusion in users. ACFs avoid this issue by adopting attack/support
relations rather than a single relation type.
      </p>
    </sec>
    <sec id="sec-5">
      <title>5. ArguCast</title>
      <p>ArguCast is an online system, available at https://argucast.herokuapp.com, accommodating
ACFs in practice as the basis for judgemental forecasting.8 We focus here on the system’s
functionalities. Note that, even though the formalisation of ACFs handles binary and
multianswer forecasting questions, ArguCast supports only binary questions currently. Also, whereas
ACFs allow for the same non-forecasting arguments to contribute to debating several forecasting
arguments, ArguCast assumes for the time being that each non-forecasting argument contributes
to debating only one forecasting argument. Thus, each ACF in ArguCast can be seen as the
composition of disjoint ACFs, one for each forecasting question.</p>
      <p>ArguCast is login-protected so all users need to register before engaging in forecasting. Users
can add their own forecasting questions (with accompanying forecasting arguments, amounting
to a positive answer to each question, as illustrated in Figure 2), or select from currently active
questions (as illustrated in Figure 3, also showing that users can search for speci c questions
and add new questions by clicking on the plus button).</p>
      <p>Figure 4 shows ArguCast’s representation of the ACF for one of the forecasting questions in
Figure 3 from the viewpoint of a single user (i.e. u U ). Note that users do not have access
to other users’ votes and predictions but they can see everything else. Note that ArguCast
8ArguCast’s user interface is implemented with React.js (https://react.dev). Storage of arguments, users, and
predictions were on a PostgreSQL database. The Web API that connects to the database and executes queries
requested from the user interface was implemented with Python’s Flask library.
supports two tree-based visualisations of each debate/ACF: a global, abstract visualisation (as
shown at the top of Figure 4) , focusing on the relations between the arguments in the ACF;
and a localised visualisation around speci c arguments (as shown in the centre of Figure 4),
showing their attackers and supporters and allowing the user to vote (cf. V ). The localised
visualisation supports the addition of supporting and attacking arguments. In both visualisations,
forecasting arguments are outlined in blue and non-forecasting arguments are outlined in grey.
The attack/support relations (i.e. A/S) between arguments are represented as red/green edges,
respectively, with a minus/plus (-/+) sign, respectively.</p>
      <p>Below the question in Figure 4, the current group forecast is shown. The group forecast
does not change in our system as of yet as the aggregation of users’ predictions (which will be
de ned in Section 6) is not implemented in ArguCast.</p>
      <p>
        Below the debate, the user has the ability to put forward their forecasting prediction (i.e. P)
for the forecasting argument using the slider. The slider ranges from [0, 100]%, which maps to
[
        <xref ref-type="bibr" rid="ref1">0, 1</xref>
        ]. The user can change their vote as they please, iteratively.
      </p>
      <p>Any user can see the debates already present in the system. However, in order to add new
arguments, cast opinions by voting, and put forward predictions, the user needs to be signed in
to their account. If a user does not have an account they can sign up with their email or Google
account.</p>
    </sec>
    <sec id="sec-6">
      <title>6. Extensions of ArguCast Frameworks</title>
      <p>The ArguCast frameworks are the base for improving forecasting systems using argumentation.
One way of doing so is to de ne notions of rationality so we can lter out irrational users when
aggregating forecasting predictions, which we will now demonstrate. Note that ltering for
rationality and the aggregation of users’ forecasts has not, as of yet, been implemented into
ArguCast system.</p>
      <p>In the remainder, unless speci ed otherwise, we will assume as given an ACF X , R, U , V, P .
We will also assume that R in this ACF is acyclic.</p>
      <p>An ACF captures the opinions of all users (in U ) involved in forecasting. We can lter out
the opinions of individual users as user QBAFs, i.e. a QBAF representing a single user’s votes
in the ACF, and then apply gradual semantics thereto for determining rationality of users by
comparing the strengths of arguments and votes/forecasting predictions.</p>
      <p>De nition 2. A user QBAF for u U is a QBAF X , A, S, u such that, for a X :
u(a) =
1 if V(u, a) = +,
0 if V(u, a) = ,
0.5 if V(u, a) is unde ned.</p>
      <p>The attacking and supporting strengths of an argument a X
as ( (A(a))) and ( (S(a))), denoted Au(a) and Su(a).
in the user QBAF are de ned
Example 2. A user QBAF for u1 for the ACF given in Example 1 would be X = {f1, d1, d2, d3},
A = {(d2, f1), (d3, d1)}, S = {(d1, f1)}, u1 (f1) = 0.5, u1 (d1) = 1, u1 (d2) = 0, and
u1 (d3) = 0.5. Using DF-QuAD the strength of arguments is (d3) = 0.5, (d2) = 0, (d1) =
c(1, 0.5, 0) = 1 1·|0 0.5| = 0.5, and (f1) = c(0.5, 0, 0.5) = 0.5+(1 0.5)·|0.5 0| = 0.75.
Then attacking and supporting strengths of the non-forecasting argument d1 is Au(d1) = 0.5 and
u(d1) = 0, respectively.</p>
      <p>S</p>
      <p>We de ne two notions of user rationality for ACFs: vote rationality, which compares the vote
of a user on any non-forecasting argument with its strength; and prediction rationality, which
compares the user’s forecasting prediction on any forecasting argument with its strength. In
the remainder, we will assume as given a user QBAF X , A, S, u for a user u U in the ACF.</p>
      <sec id="sec-6-1">
        <title>De nition 3. User u is vote rational i</title>
        <p>a X :
if V (u, a) = then
if V (u, a) = + then</p>
        <p>Au(a)
Au(a)</p>
        <p>Su (a);
Su (a).</p>
        <p>Example 3. Continuing Example 2, user u1 is not vote rational. The user agreed with d1 and the
strength of the attacking arguments is bigger than the strength of the supporting arguments (i.e.</p>
        <p>Au (d1) &gt; Su (d1)). In this instance, the vote rationality forces the user to add reasoning for the
argument d1 or put forward their opinion on d3.</p>
      </sec>
      <sec id="sec-6-2">
        <title>De nition 4. User u is prediction rational i</title>
        <p>a A :
if (a) &lt; 0.5 then P (u, a) &lt; 0.5;
if (a) &gt; 0.5 then P (u, a) &gt; 0.5;
if (a) = 0.5 then P (u, a) = [0.5
, 0.5 + ] f or some small .</p>
        <p>Example 4. Continuing Example 2, user u1 is not prediction rational. User u1’s forecasting
prediction is P (u1, f1) = 0.2 and the strength of the forecasting argument is (f1) = 0.75. In
this instance, u1 needs to change its forecasting prediction to be below 0.5 or update its vote(s) to
decrease the strength of f1. This demonstrates how prediction rationality requires that a user’s
forecasting predictions be in line with their votes.</p>
        <p>De nition 5. The ACFs are collectively rational i ( u U ) are vote rational and prediction
rational.</p>
        <p>Aggregation of forecasts requires all the agents to be collectively rational. The process of
aggregation thus uses only the forecasting predictions.</p>
        <p>
          We use a weighted aggregation function where the weights are Brier scores [
          <xref ref-type="bibr" rid="ref19">19</xref>
          ] which
represent the accuracy of each user in the previous questions that have an outcome. So, the
users with better Brier scores will have a greater in uence on the aggregated prediction. The
outcome of each question is represented by Oi { 0, 1}, where Oi = 1 if the outcome was true
and Oi = 0 if the outcome was false.
        </p>
        <p>De nition 6. Given all N outcomes as a set ({O1, ...ON }) and the corresponding forecasting
predictions for user u U ({P (u)1, ..., P (u)N }), the Brier score of u is:
Brier scores are the mean squared error of the user’s forecasting accuracy. A low bu represents
higher accuracy and a high bu represents lower accuracy.</p>
        <p>
          Then, our aggregation function is an adaptation of [
          <xref ref-type="bibr" rid="ref20">20</xref>
          ] where we also use (the negation of
the) Brier scores to obtain a weighted aggregation.
        </p>
      </sec>
      <sec id="sec-6-3">
        <title>De nition 7. The geometric mean of odds with systematic bias</title>
        <p>
          The geometric mean of odds has been shown (empirically) to outperform [
          <xref ref-type="bibr" rid="ref20">20</xref>
          ] the arithmetic
mean of odds as uncertain predictions will have less in uence. Note also that if the systematic
bias is 1 then the geometric mean of odds is similar to the arithmetic mean of odds. We will use
= 2.5 for simplicity, however in practice, the value of could be estimated [
          <xref ref-type="bibr" rid="ref20">20</xref>
          ].
        </p>
      </sec>
    </sec>
    <sec id="sec-7">
      <title>7. Conclusions and Future Work</title>
      <p>We have introduced our novel ACFs, accommodating forecasting predictions from users,
argumentative debates (as Bipolar AFs) amongst users, and votes on arguments exchanged in these
debates. We also described ArguCast, our online platform which instantiates ACFs. Then, we
de ned our notions of rational users for ACFs and showed how we can lter out irrational users
when we combine users’ predictions. We have also shown a way to combine users’ predictions
using the geometric mean of odds weighted by users’ Brier scores.</p>
      <p>
        ACFs open up numerous avenues for future work. First, we plan to implement rationality
constraints and prediction aggregation (in the forms discussed in Section 6 as well as others)
in our online system and then empirically evaluate how much the accuracy of the forecasts
improves, comparing with those de ned in [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]. Second, we will build on the fact that ACFs
provide the formal basis for further theoretical developments combining forecasting and
argumentation. For example, at the moment, ACFs only allow users to vote on non-forecasting
arguments so that we can apply rationality constraints to users. However, we would like to see
how we can accommodate votes on attack/support relations to capture the users’ beliefs on
the relations between arguments, which would allow us to extend the rationality constraints
we have introduced. Another possible theoretical development would be to include a mapping
from users to their contributed arguments to assess how this ownership a ects voting, possibly
allowing us to model users’ cognitive biases, such as con rmation bias [
        <xref ref-type="bibr" rid="ref21">21</xref>
        ], with argumentation,
as in [
        <xref ref-type="bibr" rid="ref22">22</xref>
        ]. Finally, it would be interesting to generate explanations for the combined prediction,
leveraging on argumentation’s amenability for explanation (see [
        <xref ref-type="bibr" rid="ref23 ref24">23, 24</xref>
        ] for recent surveys on
its application to explainable AI).
      </p>
    </sec>
    <sec id="sec-8">
      <title>Acknowledgments</title>
      <p>This research was partially funded by the ERC under the EU’s Horizon 2020 research and
innovation programme (grant agreement no. 101020934, ADIX), by J.P. Morgan and by the
Royal Academy of Engineering, UK, under the Research Chairs and Senior Research Fellowships
scheme. Any views or opinions expressed herein are solely those of the authors.</p>
    </sec>
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