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10-11-2016 | Original Article

# On the importance of considering heterogeneity in witnesses’ competence levels when reconstructing crimes from multiple witness testimonies

Auteurs: Berenike Waubert de Puiseau, Sven Greving, André Aßfalg, Jochen Musch

Gepubliceerd in: Psychological Research | Uitgave 5/2017

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## Abstract

Aggregating information across multiple testimonies may improve crime reconstructions. However, different aggregation methods are available, and research on which method is best suited for aggregating multiple observations is lacking. Furthermore, little is known about how variance in the accuracy of individual testimonies impacts the performance of competing aggregation procedures. We investigated the superiority of aggregation-based crime reconstructions involving multiple individual testimonies and whether this superiority varied as a function of the number of witnesses and the degree of heterogeneity in witnesses’ ability to accurately report their observations. Moreover, we examined whether heterogeneity in competence levels differentially affected the relative accuracy of two aggregation procedures: a simple majority rule, which ignores individual differences, and the more complex general Condorcet model (Romney et al., Am Anthropol 88(2):313–338, 1986; Batchelder and Romney, Psychometrika 53(1):71–92, 1988), which takes into account differences in competence between individuals. 121 participants viewed a simulated crime and subsequently answered 128 true/false questions about the crime. We experimentally generated groups of witnesses with homogeneous or heterogeneous competences. Both the majority rule and the general Condorcet model provided more accurate reconstructions of the observed crime than individual testimonies. The superiority of aggregated crime reconstructions involving multiple individual testimonies increased with an increasing number of witnesses. Crime reconstructions were most accurate when competences were heterogeneous and aggregation was based on the general Condorcet model. We argue that a formal aggregation should be considered more often when eyewitness testimonies have to be assessed and that the general Condorcet model provides a good framework for such aggregations.
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Voetnoten
1
In addition to the true/false responses, participants rated their confidence with respect to each response. This was done for an unrelated study that is not part of the present article.

2
Parameter estimates were based on 11,000 iterations, of which the first 1000 iterations were used as burn-ins and therefore discarded.

3
In estimating the statistical power, we assumed an odds ratio of 3 and a proportion of discordant pairs of .55. The odds ratio is determined by the ratio of the two cells in the 2 × 2 table in which the aggregation methods did not perform equally well.

4
To determine whether all model parameters were needed to explain the observed data, we computed the badness-of-fit Deviance Information Criterion (DIC; cf. Karabatsos & Batchelder, 2003) for the GCM. In both conditions, the most complex variant of the GCM showed the best trade-off between model fit and the number of parameters and was, therefore, used in all analyses.

5
The GCM further considers differences in guessing bias and item difficulty. However, because these parameters were not important for present purposes, we do not discuss them any further.

6
Because different combinations of θ i and δ k yield the same $$D_{ik}$$, an additional constraint on Eq. (4) is necessary (Crowther, Batchelder, & Hu, 1995). Following the procedure employed by Crowther et al. (1995) and Waubert de Puiseau et al. (2012), we therefore set δ 1  = .5 in all analyses.

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Metagegevens
Titel
On the importance of considering heterogeneity in witnesses’ competence levels when reconstructing crimes from multiple witness testimonies
Auteurs
Berenike Waubert de Puiseau
Sven Greving
André Aßfalg
Jochen Musch
Publicatiedatum
10-11-2016
Uitgeverij
Springer Berlin Heidelberg
Gepubliceerd in
Psychological Research / Uitgave 5/2017
Print ISSN: 0340-0727
Elektronisch ISSN: 1430-2772
DOI
https://doi.org/10.1007/s00426-016-0802-1

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