Fixing the stimulus-as-a-fixed-effect fallacy in forensically valid face-composite research.

Face composites from eyewitnesses’ memories are a valuable resource in tackling crime. Many studies have focused on identifying the best system to produce a nameable composite. In this article, it is described that how many of these studies do not provide reliable conclusions because they fail to treat the faces constructed as being a random factor and so make the stimulus-as-a-fixed-effect fallacy. Simulations are reported in which the statistical methodologies typically employed in these studies are performed on random data generated by a null effect. The first simulation shows that the typical analysis of variance (ANOVA) analysis in this field produces a significant effect (i.e., Type 1 error) 20% of the time. A further simulation shows that using generalized estimating equations (GEE) analysis (recently employed in this type of research) does not resolve the problem. Recommendations are made for the analysis of face-composite experiments to best evaluate and hence improve the quality of the face composites made by eyewitnesses.