Estimator and System Variables in Eyewitness Identification

A distinction between estimator and system variables is made in the eyewitness research literature between two categories or types of variables that influence the accuracy of eyewitness accounts. System variables are those that are (or can be) under the control of the justice system, whereas estimator variables cannot be controlled by the justice system. Examples of system variables include factors such as the instructions given to eyewitnesses prior to their viewing a lineup or the number of people who are used in a lineup. Examples of estimator variables include factors such as how good a view the eyewitness had of the perpetrator during the crime or whether the witness and perpetrator were of the same or different race. The estimator versus system variable distinction tends to be tied to a temporal unfolding of events, in the sense that events that occur before or during the witnessing experience are necessarily relegated to estimator variable status whereas system variables begin to come into play later, once the investigation is under way. There is no presumption in the estimator variable versus system variable distinction that one category of variables has more impact on eyewitness accuracy than the other. Nevertheless, this distinction, first articulated in 1978 by Gary L. Wells, has tended to result in a higher premium being placed on system variables because these can be used to help minimize eyewitness errors in actual cases, whereas estimator variables can only be used to postdict how the variables might have influenced the eyewitness.

The study of system variables has generally been tied to policy-related recommendations on ways to improve how crime investigators interview eyewitnesses and on ways to improve how lineups are constructed and conducted. The study of estimator variables, in contrast, has more often been tied to the development of expert testimony that can assist triers of fact (e.g., judges, juries) in deciding whether to accept the testimony of an eyewitness as having been accurate or mistaken. In fact, however, system variables are as relevant to expert testimony as are estimator variables, and in recent years, it has become more apparent that estimator variables and system variables are not independent. In general, the impact of system variables is likely to depend somewhat on the levels of the estimator variables. An obvious example of this dependence is when the estimator variables are highly favorable to the existence of an extremely deep, solid memory. If memory is strong enough, system variables would not likely have much impact. For instance, system variable research shows that it is critical for eyewitnesses to be warned prior to viewing a lineup that the actual perpetrator might not be present, because the absence of such a warning leads eyewitnesses to select someone from a lineup even if the actual perpetrator is not present. However, if the eyewitness's memory is strong enough (e.g., [Page 258]attempting to pick one's own mother from a lineup), the presence or absence of this warning is of little consequence. Hence, a complete understanding of eyewitness performance clearly requires research on both system and estimator variables.

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