Lineup Size and Bias

Lineup Fairness Evaluation

To determine whether the two basic rules have been adhered to, lineup evaluation research participants who have not seen the offender are asked to guess which lineup member is the suspect on the basis of a brief physical description (e.g., the description originally given by the witness to the police) or with no information at all about the appearance of the offender.

Lineup Bias

Bias is bidirectional—it can make the suspect more likely, or less likely, to be chosen by a witness who has no or very little specific memory of the offender. It is defined as a statistically reliable tendency for a [Page 458]suspect to be chosen from a lineup at a rate different from that expected if the choice were made by chance alone (i.e., a random pick of a lineup member). The proportion of lineup evaluation research participants able to pick the suspect from the lineup is a measure of lineup bias.

Lineup Size

Nominal size is the number of persons presented in a lineup. Effective size, as noted earlier, is the number of persons in the lineup who are effective choice alternatives for a witness who has little information or memory about the actual offender or for a witness viewing a lineup in which the suspect is actually innocent (i.e., is not the offender). It is thought of as a reduction of the nominal size to a value that better represents the “true” number of plausible foils, or innocent persons in a police lineup (i.e., nominal size ≥ effective size). The index E, is a measure of the effective size of a lineup.

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