Effective Size and Defendant Bias in Eyewitness Identification Lineups

• NCJ Number: 81338
• Journal: Law and Human Behavior Volume: 5 Issue: 4 Dated: (1981) Pages: 299-309
• Author(s): R S Malpass
• Date Published: 1981
• Length: 11 pages
• Publication Format: Article
• Publication Type: Statistics
• Language: English
• Country: United States of America

Annotation

Defendant bias and lineup size are different aspects of fairness in eyewitness identification lineups; the degree to which bias towards or away from the defendant is present should be determined on the basis of the lineup's effective size.

Abstract

The effective size of a lineup represents the degree to which the lineup presents mock witnesses with fewer effective choice alternatives than the number of individuals in the lineup: the greater the degree to which foils fall below nominal chance expectations and the more foils which fall below it, the smaller the effective size of the lineup. The bias towards or away from the defendant is the difference between the observed proportion of identifications made of the defendant and the proportion of defendant identifications expected by chance (e.g., an unbiased six-person lineup would have the defendant identified by chance one-sixth of the time). Effective size rather than nominal size of the lineup should be used as the basis for calculating chance expectation. Conventional statistical methods can determine whether the difference between the observed proportion of identifications is significantly different from the adjusted chance expectation, given the size of the sample of mock witnesses. The effective size concept can be applied before an identification has taken place to prepare a lineup which will survive criticism and which is demonstrably fair. The procedure would probably be utilized mostly in important cases because of the expense and time involved. Its more frequent application may be as an instrument of the defense in discrediting eyewitness testimony or of the prosecution as a means of bolstering a case by showing the fairness of a lineup. Calculations are explained that will produce an effective-size index and ascertain defendant bias, based on data obtained from lineup choices by a number of (ca.100) mock witnesses. Procedures are detailed for enlisting, and instructing mock witnesses, presenting them with live or photographic lineups, and obtaining the required data. Tables, footnotes, and 10 references are given.