Police evaluation of evidence: statistical format and evidence type

Abstract

Abstract Police must properly evaluate evidence to make consequential decisions. Erroneous evaluations of evidence could begin a process that leads to wrongful convictions through faulty guilt judgments. This study assessed the extent to which police accuracy in judging suspect guilt varied by the statistical format of evidence strength and the type of evidence. We recruited police participants (n = 209) from CloudResearch and randomly assigned them to a 3 (evidence format: LR vs. RMP vs. neutral) x 3 (evidence type: DNA vs. fingerprint vs. eyewitness) between-subjects factorial design. Police participants read a case scenario in which they investigated a murder case and evaluated suspect guilt based on a piece of evidence. Prior odds of guilt were controlled for as 1 in all conditions. In general, police under-weighed evidence in the RMP format and were more accurate at judging suspect guilt when the evidence was in the LR format and when the evidence was inconclusive. The effect of evidence format was particularly strong for DNA evidence. Additionally, police participants’ responses did not differ between different guilt measures. Overall, police showed inconsistencies in their guilt judgments when evaluating evidence with different statistical formats and evidence types.