=Paper=
{{Paper
|id=Vol-1751/AICS_2016_paper_19
|storemode=property
|title=Noise in Reasoning as a Cause of the Conjunction Fallacy
|pdfUrl=https://ceur-ws.org/Vol-1751/AICS_2016_paper_19.pdf
|volume=Vol-1751
|authors=Rita Howe,Fintan Costello
|dblpUrl=https://dblp.org/rec/conf/aics/HoweC16
}}
==Noise in Reasoning as a Cause of the Conjunction Fallacy==
https://ceur-ws.org/Vol-1751/AICS_2016_paper_19.pdf
Noise in Reasoning as a cause of the Conjunction Fallacy
Rita Howe, Fintan Costello
University College Dublin, Dublin, Ireland
Rita.Howe@ucdconnect.ie
Fintan.Costello@ucd.ie
Abstract. The conjunction fallacy occurs when people judge a conjunction
A&B as more likely than a constituent A, contrary to the rules of probability
theory. We describe a model where this fallacy arises purely as a consequence
of noise and random error in the probability estimation process. We describe an
experiment testing this proposal by assessing the relationship between fallacy
rates and the average difference between conjunction and constituent estimates
(in the model, the smaller this difference, the more likely it is that the conjunc-
tion fallacy can occur due to random error), and by assessing the degree of in-
consistency in people’s conjunction fallacy responses for repeated presentations
of the same probability questions (in the model these responses should tend to
be inconsistent due to random error, especially in cases where the average dif-
ference in estimates is low). Experimental results support both these predic-
tions.
Keywords: probability estimation; conjunction fallacy; cognitive science
1 Introduction
Research into human reasoning under uncertainty has uncovered many surprising
results about how humans estimate the probabilities of uncertain events. One of the
most surprising of these findings is the conjunction fallacy, which arises when sub-
jects judge some conjunction of events A&B to be more likely (more probable) than
one of the constituent events of that conjunction, A. This constitutes a violation of the
conjunction rule of probability theory, which requires that P(A&B) ≤ P(A) and
P(A&B) ≤ P(B) must always hold (simply because A&B cannot occur with A or B
themselves occurring). The conjunction, A&B, under the probabilistic laws, cannot be
more likely than the single constituent A, thus when a participant chooses the con-
junction A&B as more probable, they are committing a fundamental violation of ra-
tional probabilistic reasoning.
This finding was first uncovered by Tversky and Kahneman [1]. They found that
when presented with the now famous Linda problem, over 80% of their participants
made an erroneous judgement. The Linda problem is as follows:
adfa, p. 1, 2011.
© Springer-Verlag Berlin Heidelberg 2011
Linda is 31 years old, single, outspoken, and very bright. She majored in philoso-
phy. As a student, she was deeply concerned with issues of discrimination and so-
cial justice, and also participated in anti-nuclear demonstrations.
A. Linda is a bank teller
B. Linda is a bank teller and is active in the feminist movement
The conjunction fallacy is ubiquitous in this field of cognitive science. Tversky and
Kahneman’s widely replicated results were taken as an indication that humans do not
reason in a normative fashion – that is, they don’t apply probabilistic rules to real-life
contexts. Instead, they reasoned that people employ a "representativeness heuristic"
when they encounter probability problems such as Linda. Under this theory, the falla-
cy occurs as the person described in the conjunction is more representative of the
information presented in the character sketch.
However, research has called the validity of the representativeness account into
question. Experiments that manipulated class inclusion, for instance, demonstrated
that the fallacy occurs regardless of whether the conjunction is representative or not
[2]. Formal probabilistic models have sought to show that a range of biases can be
explained as a function of probabilistic reasoning instead of a heuristic process. The
majority of these look towards quantum probability theory and standard probability
theory [3, 4].
In this paper we describe an experiment testing two predictions of one of these ac-
counts: the probability theory plus noise model [4]. This model assumes that people
estimate probabilities using a fundamentally rational process which is, however, sub-
ject to the systematic biasing effects of noise in the reasoning process. This model
makes two predictions about the conjunction fallacy. First, this model predicts that
fallacy rates for a conjunction A&B should be reliably related to the average differ-
ence between estimates A&B and A (the closer these estimates are, the greater the
chance that random noise will move one individual estimate for A&B above that for
A, producing a conjunction fallacy response). Experimental results strongly support
this prediction. Second, this model assumes that conjunction fallacy responses will be
inconsistent in certain cases, due to random noise (with a given participant sometimes
producing the fallacy but sometimes not producing the fallacy, for the same conjunc-
tion A&B). Experimental results confirm that this inconsistency occurs in just the
cases predicted by the model.
1.1 Background
Many of the language based experimental manipulations of the fallacy focus on re-
sponse mode or framing effects. Response modes typically examine the difference in
fallacy rates when the question is posed in a probability or frequency format, and
much research has shown that the probability format inflates the fallacy rates in com-
parison to frequency formats, sometimes quite dramatically [5].
Other manipulations focused on the role of the "frame" in the scenario. The frame
is the character sketch or additional information supplied to the participant. Framing
descriptions are usually designed so there is a strong confirmation link between one of
the constituents in the conjunction and the frame. Participants in experiments that
have framing descriptions tend to have a high rate of the conjunction errors [1], [6],
[7]. Multiple studies have found that the conjunction fallacy occurs reliably even in
studies where the participants saw no character sketch or description of an occurrence,
though fallacy rates tend to be somewhat lower in these studies than in studies using a
framing scenario [7, 8, 9]. While the framing description is the not critical component
for eliciting the fallacy, it does provide a mediating factor for the extent to which the
fallacy occurs. Consistently, across the research, higher rates of the conjunction falla-
cy occur when there is a framing description. Not only do participants commit the
fallacy at higher rates when they are provided with framing information, they also
commit the fallacy at higher rates if they are given analogous probability based in-
formation instead of a character sketch. In both those cases, the participants commit-
ted the fallacy at about twice the rate as when there was no frame [8]. The inclusion
of “filler items” in frames increases both the standard error and the likelihood that the
fallacy will be committed [10]. Fallacy rates will decrease when the frame is de-
creased but to our knowledge, no research has specifically investigated whether the
opposite effect will occur.
1.2 Noise
However, while fallacy rates are generally quite high, a frequent observation
among this research is that a small number of participants do not seem overly suscep-
tible to the fallacy. In addition, over a number of conjunction problems, participants
rarely have 100% error rates. Stanovich and West recognized that individuals can
differ greatly on their performances on cognitive bias eliciting tasks. They found that
subjects with higher cognitive ability were disproportionally likely to avoid commit-
ting a number of cognitive biases including the conjunction fallacy [11]. Hilbert pro-
posed a theoretical framework of noisy information processing to explain a number of
cognitive biases [12]. Under this framework, memory based processes convert obser-
vations stored in memory into decisions. By assuming that these processes are subject
to noisy deviations and that the noisy deviations are a generative mechanism for falla-
cious decision making, the framework provides an explanation for a number of well-
known cognitive biases including the conjunction fallacy. Previously, weighted mod-
els were popular as a means to explain the range of results that were consistently ob-
served in fallacy research [13, 14]. However, these were limited in the scope of results
that they could predict. A more successful iteration of these weighted models is the
configural weighted average (CWA) model [15]. This sophisticated weighted model
includes a “noise component” that randomly disturbs probability judgements. Fisher
and Wolfe tested the predictions of a number of “noisy” models of reasoning – both
normative and non-normative - and found that many of the normative noisy model
predictions were good fits for a range of experimental results on cognitive biases [16].
The Probability Theory Plus Noise model. A survey of the conjunction fallacy liter-
ature reveals that for standard conjunction problems, the fallacy rate is highly variable
(ranging from ~10% to ~90% for different problem scenarios). Costello and Watts
[15], [17] proposed an alternative account (the probability theory plus noise model)
which can account for this variability in occurrence of the conjunction fallacy. In this
model people do reason in a normative fashion according to probability theory, but
are subject to random error in the reasoning process. The reasoner’s decision making
processes, which are memory based, reliably apply the conjunction rule during the
probability estimation process, but random noise causes fluctuations in judgement that
sometimes lead to the subjective probability of a conjunction exceeding the subjective
probability of the constituent. Costello and Watts showed that a simulation imple-
menting this model produced a wide range of fallacy rates (from less than 10% to
close to 70%) and produced conjunction fallacy rates for a given set of materials that
closely matched those seen in experimental studies for the same materials [17].
The model is built on the assumption of a rational reasoner with a functionally
normal long-term memory who is subject to a minimal amount of transient random
noise. Long-term memory contains m number of episodes where each recorded epi-
sode, i, has a flag that is set to 1 if i contains event A and is set to 0 if it does not. The
random noise is represented by d, the small probability that when the flag is read an
incorrect value for it is returned. Each event is assumed to have a minimal representa-
tion in that it is represented by a flag. Each flag has the same probability of being read
wrong. Given this, a randomly sampled item could be counted as an instance of A in
two different ways; if it is an instance of A and is counted correctly or if it isn’t an
instance of A and is counted incorrectly. The probability of an item being an instance
of A and being counted correctly, then, is P(A)(1- d), where P(A) is the chance of an
item being an instance of A and (1- d) the is chance of any item being counted cor-
rectly. The probability of an item being not A and being incorrectly read as A is (1 –
P(A))d, where (1-P(A)) is the chance of an item not being an item of A and d is the
chance of an item being read incorrectly. Combining these two expressions, the ex-
pected value or average for a noisy estimate of P(A) is
〈𝑃𝐸 (𝐴)〉 = P(A)(1 – d) + (1 – P(A)) = (1 -2d)P(A) +d (1)
With individual estimates PE(A) being expected to vary independently around this
expected value. The model predicts this bias for noise for all types of events, howev-
er, it will be more pronounced for complex events. This is due to the process of count-
ing instances of A&B being more complex than counting instances of A as there is
more scope for error. To reflect this increased chance of error, the model assumes a
random error rate of d + Δd for complex events. The expected value for conjunctive
estimates is:
〈𝑃𝐸 (𝐴&𝐵)〉 = (1 – 2[d + Δd])P(A&B) + [d + Δd] (2)
The model proposes that the conjunction fallacy occurs as the individual estimates
for A and A&B vary around their expected values as a result of transient noise. This
random variation produces a situation where some PE(A) < PE(A&B) and the con-
junction fallacy will result. The model predicts that the closer PE(A) and PE(A&B)
are to each other, the higher the likelihood of a fallacy occurring – it predicts that the
rate of fallacious responses will increase with the difference between the average
estimates, which is given by the expression:
〈𝑃𝐸 (𝐴&𝐵)〉 – 〈𝑃𝐸 (𝐴)〉
= (1 -2[d +Δd])P(A&B) + [d +Δd] – (1 – 2d)P(A) – d
= (1 -2d)[P(A&B) – P(A)] +Δd[1 - 2P(A&B)] (3)
When this difference is negative (and so the expected value for estimates of
A,〈𝑃𝐸 (𝐴)〉, is greater than the expected value for estimates of A&B, 〈𝑃𝐸 (𝐴&𝐵)〉), the
fallacy rate is predicted to be less than 50% (because random error will 'move' the
estimate for A&B above that for A less than 50% of the time. When this difference is
positive, however (and so the expected value for estimates of A, 〈𝑃𝐸 (𝐴)〉, is less than
the expected value for estimates of A&B, 〈𝑃𝐸 (𝐴&𝐵)〉), the fallacy rate is predicted to
be greater than 50% because random error will 'move' the estimate for A&B below
that for A less than 50% of the time. This difference will be positive when
Δd[1 – 2P(A&B)] > (1 -2d)[P(A) – P(A&B)] (4)
From this expression, we see that the average estimate for A&B can be higher than
the average estimate for constituent A when Δd is positive; when 1 - 2P(A&B ) > 0
(that is, when P(A&B) < 0.5); and when P(A) - P(A&B) is small (that is, when
P(A&B) is close to P(A)). When these three requirements hold, the conjunction falla-
cy should occur more than 50% of the time for individual estimates. In addition to
this, the more positive the difference between the terms, PE(A&B) and PE(A), the
greater the occurrence of the conjunction fallacy. When the difference between these
terms is negative, the model predicts a conjunction fallacy rate of less than 50%.
Note that, since in this model the conjunction fallacy is produced by random varia-
tion in probability estimates for P E(A&B) and PE(A), a fundamental prediction of this
model is that the occurrence the conjunction fallacy will be inconsistent. Specifically,
if we ask the same participant to estimate the same probabilities PE(A&B) and PE(A)
at two separate times, this model predicts that they may produce the conjunction fal-
lacy at one but not at the other time (due to random variation in estimates). The more
negative the difference 〈𝑃𝐸 (𝐴&𝐵)〉 – 〈𝑃𝐸 (𝐴)〉, the more likely it is that the same par-
ticipant will not produce the fallacy at either time (random error being unlikely to
‘move’ estimates enough to produce the fallacy). The more positive the difference
〈𝑃𝐸 (𝐴&𝐵)〉 – 〈𝑃𝐸 (𝐴)〉, the more likely it is that the same participant will produce the
fallacy at both times. The closer the difference 〈𝑃𝐸 (𝐴&𝐵)〉 – 〈𝑃𝐸 (𝐴)〉 is to 0, the
more likely inconsistent fallacy responses will be (random error causing a fallacy at
one time, but not at the other).
Because this model assumes a frequency representation for people’s judgments of
probability, it gives a natural account for the observed pattern of lower fallacy rates
when materials are presented in terms of frequencies rather than probabilities (the
frequency representation is closer to the representation used in people’s probability
estimates, and so is less subject to random error and less susceptible to the conjunc-
tion fallacy). The model also gives a natural account for the higher fallacy rates seen
when probabilities are estimated relative to a framing scenario: that scenario introduc-
es more information to the probability estimation process, and so increases the chance
of random error (and so the chance of the conjunction fallacy). More generally, in
the noise model, probability estimates for longer statements should be more variable
than estimates for shorter statements (simply because the long statements contain
more information and so more opportunity for random error to occur). If people’s
probability estimation is subject to the type of noise assumed in the noise model, we
should see this difference in variability for probability estimates for statements of
different lengths.
1.3 Predictions
We can summarize the probability theory plus noise model's predictions as fol-
lows: First, the rate of conjunction fallacy occurrence relative to A (that is, the rate at
which people judge P(A&B)>P(A) ) should follow the average difference in estimates
for P(A&B) and P(A), and should be greater than 50% when that difference is posi-
tive. Second, participants should show inconsistency in conjunction fallacy produc-
tion, with the same participant producing the fallacy at one time T1 but not at another
time T2, for some conjunctions A&B. This inconsistency in fallacy responses should
be most frequent when the difference P(A&B)-P(A) is small. When the difference is
negative (when P(A&B)-P(A)<0) the model predicts consistent avoidance of the fal-
lacy (no fallacy at either times); when the difference is positive (when P(A&B)-
P(A)>0) the model predicts consistent production of the fallacy (fallacy occurrence at
both times).
The model also makes some predictions about variation in probability estimates: it
predicts that the degree of variation in probability estimates for conjunctive state-
ments A&B should tend to be greater than the degree of variation in estimates for
single statements A or B. It also predicts that the degree of variance in estimates for
long statements should be higher than the degree of variance for short statements.
We test these predictions below.
2 Experiment
2.1 Design
The experimental materials consisted of 8 probability estimation problems, pre-
sented to participants twice at two time periods, T1 and T2. Each of the problems was
a simple scenario describing an individual (in the style of the Linda problem) fol-
lowed by four statements regarding that person. There were three different blocks of
problems. The first block consisted of 4 scenarios with P(A), P(B), P(A&B), and
P(AorB) estimates, the second block consisted of 2 scenarios with P(A), P(A&B),
P(C&D), and P(AorC) problems. The final block consistent of 2 scenarios with P(B),
P(A&B), P(C&D), and P(BorC) estimates. There were four "fillers" which asked the
participants to estimate the probabilities of P(A), P(B), P(C), and P(D). The filler
scenarios also asked for probability judgements but the statements contained no con-
junctions or disjunctions and each of the four were independent of the others.
In addition to this, an extra manipulation was introduced: some of the probability
statements were presented in "short" versions and some in "long" versions. The short
versions mirrored the classic materials used in past research e.g. participants estimat-
ed the probability that "Linda is a boxer and works in an art gallery", while the long
versions keep the semantic content as close to the short versions as possible, while
increasing the overall sentence length in the probabilities to be estimated e.g. “Linda
is a very successful boxer in the amateur welterweight division and a curator respon-
sible for post minimalism modern art in an art gallery”. No claim is being made that
the long and short versions were semantically identical. Rather, they were kept as
close as possible to allow for comparison: the rationale for the manipulation being
that the longer statements should increase the variability of participant’s responses.
Each participant saw exactly the same materials twice, at T1 and T2.
In total, the participants gave probability estimates for 128 items. To control for a
memory effect, the order of the scenarios was randomly generated for both T1 and T2
testing. Within the scenarios, the order of the statements was also randomly generated
for each trial. There was a 50/50 split for short and long scenarios for each participant.
2.2 Method
40 female and male students were recruited from the student body. They were giv-
en a brief description of their task (a two-part estimation task on novel events) and
informed that there was no time limit on task completion. The participants were asked
to provide probability judgements for statements following a description of a fictional
person. Once they had completed part one of the task, they were given part two to
complete. At no stage did they have access to their responses to the previous part.
2.3 Results
Of the 40 participants who completed the task, 3 were excluded from the final
analysis as they did not complete a sufficient number of questions (missing more than
10% of responses). As in other experiments, many of the participants had high con-
junction fallacy rates.
Time. For T1, 97% of the participants had at least one conjunction fallacy in their
responses. The mean fallacy rate was 45% for the group, while the short condition
had 39% mean fallacy rate, the long condition had a higher rate of 51%. In T2, 92%
of the participants committed the conjunction fallacy at least once with a mean fallacy
rate of 44%. In the short condition, the mean fallacy rate was 41% while the fallacy
occurrences were slightly higher for the long condition, with a mean fallacy rate of
47%.
Difference in Probability Estimates. The conjunction fallacy plus noise model
predicts conjunction fallacy rates for A&B versus A to be greater than 50% for cases
where the P(A&B) - P(A) values are greater than zero. P(A&B) – P(A) values less
than zero should produce fallacy rates that are less than 50% and P(A&B) – P(A)
values that are close to zero should produce fallacy rates around 50%. The same pre-
dictions should also hold for P(A&B) – P(B).
We tested these predictions by calculating the average values for each of the con-
stituents and complex events across time and compared these values to the observed
fallacy rates for the scenarios. Table 1 above displays the results of the analysis for
the short scenarios. The prediction was tested the same way for the long scenarios.
These results are displayed in Table 2. As predicted by the model, the cases where a
positive difference between P(A&B) – P(A) or P(A&B) – P(B) was observed, the
average fallacy rate was above 50%. The estimate differences that were zero or close
to it, produced fallacy rates around 50%. Cases that had negative differences pro-
duced low fallacy rates, ranging from 2% to 20%.
There was a strong positive correlation between difference in estimates and fallacy
rates, r = 0.93, p < 0.0001. The strong positive correlations between estimate differ-
ence and fallacy rate held for both long and short versions of the materials. The esti-
mate difference from the short condition had a strong positive correlation with fallacy
rates, r = 0.95, p < 0.0001 as did the difference in estimates for the long condition, r =
0.95, p < 0.0001.
Table 1. Observed conjunction fallacy rates for difference in conjunction and constituent
estimate for short scenarios. The average difference between the constituent and the
conjunction was calculated for each of the short scenarios. The results are sorted in descending
order. Overall, there was a significant positive correlation between fallacy rates and average
difference. Note: Participants did not give P(A) estimates for scenario 7 and P(B) estimates for
scenario 8
Scenario P(A^B) vs P(A) Scenario P(A^B) vs P(B)
Fallacy Fallacy
Difference rate Difference rate
8 -0.121 20.3% 1 -0.423 5.3%
2 0.088 52.8% 3 -0.395 2.6%
4 0.141 69.4% 2 -0.389 2.8%
1 0.166 68.4% 4 -0.279 8.3%
3 0.189 78.9% 7 -0.167 2.7%
Table 2. Observed conjunction fallacy estimates for difference in conjunction fallacy estimates
for long scenarios. The average fallacy rate had a significant positive correlation with average
difference. Note: Participants did not give P(A) estimates for scenario 5 and P(B) estimates for
scenario 6.
Scenario P(A^B) - P(A) Scenario P(A^B) - P(B)
Fallacy Fallacy
Difference rate Difference rate
6 0.056 44.6% 2 -0.378 2.6%
4 0.102 55.3% 3 -0.374 2.8%
1 0.102 61.1% 4 -0.352 10.5%
2 0.129 73.7% 1 -0.306 2.8%
3 0.159 66.7% 5 -0.058 17.6%
Inconsistent Fallacy Production. One of the marked observations of these results is
the variable performance of participants on the fallacy problems. While some
participants were consistent in their responses to a particular scenario, either reliably
reproducing the fallacy on both occasions or not producing the fallacy at all for that
problem, there was a large number of inconsistent responses where the fallacy was
produced by a participant on one presentation of a given scenario (at T1 or T2), but
not produced by that participant on the other presentation of exactly the same
scenario. These inconsistent fallacy responses made up 21% of the total responses to
conjunctions. To test the noise model’s predictions about this inconsistency, the rate
of occurrence of these inconsistent fallacy responses was compared to the average
difference in conjunction and constituent estimates for that scenario. Recall that the
probability theory plus noise model predicts that these inconsistent fallacy responses
should be most frequent when this difference in estimates is close to 0. Figure 1
graphs the frequency of occurrence of this inconsistency in the conjunction fallacy.
Of the inconsistent fallacy responses, nearly half (45%) had average differences in
estimates that fell between -0.05 and +0.05, as predicted by the model. As the graph
shows, these inconsistent fallacy responses were distributed approximately
symmetrically around 0; cases with no fallacy responses overwhelmingly had
negative average differences (fell on the left side of the graph) and cases with where
two fallacy responses overwhelmingly had positive average differences (fell on the
right side of the graph). Again, this supports the predictions of the noise model.
Variability. One of the predictions of the model is that complex events should be
more variable than single events. Table 3 below displays the overall standard devia-
tions (SDs) in the participants' estimates for P E(A), PE(B), and PE(A&B) for each of
the scenarios regardless of length type. Comparisons of the variability in the conjunc-
tive events versus the simple events showed higher variability in the complex events
in 88% of the cases. A similar pattern was observed in the short and long scenarios
where the estimates for constituents A and B tended to be less variable than the con-
junction, A& B. Comparisons showed that for both types, the conjunction was more
variable in 83% of the occasions. F-tests for equality of variance were then carried
140
120
FREQUENCY 100
80
Zero
60
One
40 Two
20
0
-1 -0.5 0 0.5 1
AVERAGE DIFFERENCE IN ESTIMATES
Fig. 1. Participants' consistency in their production of the conjunction in T1 and T2 were calcu-
lated. There were 296 possible fallacy responses over T1 and T2, recorded as ‘zero’ if a partic-
ipant produced no fallacy at either time, ‘one’ if the produced the fallacy at one time but not the
other (an inconsistent response) and ‘two’ if they produced the fallacy at both T1 and T2. For
each fallacy response the corresponding average difference in estimates, PE(A&B) – PE(A), was
calculated; fallacy responses were placed into 'bins' from -0.95 to +0.95 in steps of 0.1 based on
this difference. The graph shows the frequency of each conjunction fallacy response in each
bin. The model predicts that cases of zero fallacy should be associated with a negative differ-
ence in estimates, cases of inconsistent fallacy production should produce differences close to 0
and cases of the fallacy occurring on both occasions should be associated with a positive differ-
ence. The graph supports the model's predictions.
out to compare the conjunctive and constituent variability.
Overall, the F-test for equality of variance (df1 = df2 = 36) was statistically signif-
icant in 33% of the occasions (p < 0.05) with another 17% of the sample approaching
significance (p = 0.10).
For the short scenarios (df1 = df2 = 36), the F-test for equality of variance was sta-
tistically significant in 40% of the cases (p < 0.05) with a further 10% approaching
significance. For the long scenarios (df1 = df2 = 36), the F-test for equality of vari-
ance was statistically significant in 30% of the cases (p < 0.05) with a further 20% of
comparisons were approaching significance (p = 0.10).
A final prediction for the model is that the degree of variance should be higher in
the estimates for the long statements than the shorter statements. We carried out a
similar analysis to see if there was any difference in variance between the correspond-
ing long and short statements.
For this, the data from scenarios 1 to 4 was used as the participants has either seen
a long or short version of each and their probability estimates for the long and short
Table 3. Standard deviations in estimates for long and short scenarios for constituents A, B
and for the conjunction A&B. SDs for the constituents were reliably lower than SDs for the
conjunctions. F-tests for equality of variance found a number of constituents that had a statisti-
cally significant difference in variability relative to their conjunction.
Scenario Long Short
SD of SD of SD of SD of SD of SD of
PE(A) PE(B) PE(A^B) PE(A) PE(B) PE(A^B)
1 19.34 22.40 18.41 17.74* 16.49* 27.47
2 19.73* 19.00* 25.06 17.40 18.48 18.59
3 20.09 19.53 20.32 20.02 18.10* 22.60
4 22.75 28.45 23.44 22.12 24.96 23.49
5 - 19.86* 25.29 - - -
6 22.54 - 23.50 - - -
7 - - - - 20.64 20.84
8 - - - 27.20* - 21.44
* p < 0.05
versions of each statement could be compared with each other. Comparisons of the
average variance for PE(A), PE(B), and PE(A&B) for those scenarios showed that the
long versions were more variable than the corresponding short version in 75% of the
cases. F-tests of equality of variance (df1 = df2 = 36) was statistically significant in
33% of the comparisons between the long and short versions of the scenarios.
3 Discussion
The results of this experiment support the probability theory plus noise model’s
proposal that the conjunction fallacy is a consequence of random error in probability
estimation (with greater random variation in conjunctions than constituent events).
First, conjunction fallacy rates were strongly predicted by the average difference be-
tween conjunction and constituent probability estimates (as we would expect if the
fallacy occurs due to random variation in these estimates: the smaller the difference
between estimates, the greater the chance that random variation would ‘move’ indi-
vidual estimates to produce a conjunction fallacy response). Second, variability was
typically higher for estimates for a conjunctive probability than in estimates for a
constituent probability, as assumed in the probability theory plus noise model. Third-
ly, estimates for the longer probability statements were typically more variable than
estimates for the corresponding short statements, as predicted by the model. With a
small sample size it is difficult to produce a clear picture of the variability of the esti-
mates but future research will look towards addressing this issue. Finally, conjunction
fallacy responses were inconsistent (with the same participant producing the fallacy
for one presentation of a given scenario but not in another presentation), just as we
would expect if the fallacy is a consequence of random error. This inconsistency was
most frequent when the average difference between estimates was small, just as pre-
dicted by the probability theory plus noise model.
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