Communicating error and expertise in forensic expert testimony

Abstract

Forensic comparison evidence, such as fingerprints, plays an important role in criminal investigations and proceedings. For more than 100 years, forensic examiners were permitted to testify that two prints, marks, or samples matched to the exclusion of all other people, with some even claiming that their decisions were infallible. However, numerous wrongful identifications have demonstrated that these claims of infallibility simply are not true. In fact, forensic science testing errors and false or misleading forensic testimony are two common factors leading to the wrongful conviction of innocent individuals. As a result of these misidentifications, forensic expert testimony is now beginning to reflect the uncertainty of forensic comparison evidence but the method in which experts should communicate uncertainty is under debate. Many have supported the use of likelihood ratios-which communicates the probability of an observation given two competing hypotheses: that the two samples share the same source (H1), or that the two samples originate from different sources (H2). However, others have argued that communicating a specific likelihood ratio is a futile endeavour when considering that the probability of a false positive (e.g., mislabelling of samples) is far greater. The overall aim of this thesis is to introduce a diagnostic information approach to communicating forensic expert testimony, which draws on signal detection theory to convey examiners' accuracy and error. Specifically, I aim to a) determine people's existing beliefs about accuracy and error in forensic science, b) determine whether likelihood ratios affect sensitivity to exculpatory evidence, such as an alibi, and c) compare the likelihood ratio approach to the diagnostic information approach to determine which results in better sensitivity to the strength of the evidence. There are three main empirical chapters of this thesis. In Chapter 2, I explore laypeople's perceptions of error and involvement of human judgment in forensic science, both in terms of the stages of the forensic science process (e.g., collection, storage) and for specific forensic techniques. I find that people do not necessarily have blind faith in forensic sciences, as participants believed that there was quite a high likelihood of error, as well as a high level of human judgment involved, in the forensic science process. Furthermore, participants had lower perceptions of accuracy for specific forensic techniques than expected, ranging from 65.18% for document analysis to 89.95% for DNA analysis.