The Effects of Coding Bias on Estimates of Behavioural Similarity in Crime Linking Research of Homicides

Abstract

This study explored whether a coding bias due to knowledge of which crimes have been committed by the same offender exists when behavioural variables are coded in serial murder cases. The study used an experimental approach where the information given to the participants (N = 60) concerning correct linkages between a number of murder series was manipulated. The participants were divided into three different groups (n = 20 in each). These three groups received correct, incorrect, or no information about the linked series prior to the coding. The results showed that there is no clear evidence to support the hypothesis of a bias in the coding. The risk of expectancy effects and suggestions on how to minimise them in behavioural crime linking research were discussed, and suggestions on how to improve the validity of possible future replications of the experiment were given. The practical implications of expectancy effects on behavioural crime linking decisions for the justice system were also discussed. Copyright © 2012 John Wiley & Sons, Ltd.

Testing the Assumptions of Crime Linkage with Stranger Sex Offenses: A More Ecologically-Valid Study

“…However, a recent study by Pakkanen, et al (2012), found that knowledge of the membership of an offense to a series did not appear to bias the coding of the behavioral data.…”

The practice of crime linkage: A review of the literature

“…As well as assessing the overall accuracy of linkage decisions made, the efficacy of particular parts of the linkage process can also be assessed. Pakkanen et al's () study investigated whether prior knowledge of links between cases would create a bias as to the similarity perceived in an offender's behaviour across a series, when participants coded the information in each case as present, absent, or missing. This is an important research question for the CLA scenario where the police have already put forward an opinion as to whether they consider a group of crimes to be linked before the analyst and then conducts their analyses.…”

“…Furthermore, all of the tasks in these studies are much simplified in comparison with the real‐life process as undertaken by practitioners. For example, Pakkanen et al () used edited summaries of murder case transcripts, which would not be reflective of the many case papers generated in a murder investigation. As a consequence, the results of such studies will have limited applicability to real‐world practice.…”

“…Another area associated with how crime linkage is conducted is the notion of cognitive bias. The idea that bias may exist in the coding of offences has been investigated in one study (Pakkanen et al, ), and in another, analysts mention their awareness of this issue (Burrell & Bull, ). Despite the potential for cognitive bias to influence the results of linkage, however, this area has also received very little academic attention.…”

“…In some countries, it is also used to support prosecutions in court (Labuschagne, , ; Pakkanen, Santtila, & Bosco, ). Using offender behaviour to link crimes can be advantageous where more traditional linkage methods are expensive and time‐consuming (Pakkanen, Zappalà, Grönroos, & Santtila, ), or where there is limited or no physical forensic evidence (Grubin, Kelly, & Brunsdon, ; Labuschagne, ).…”