Be A Juror

(Editor:   Barney Beins)


Eyewitness testimony research has informed the federal government's guidelines for competent use of eyewitness evidence in criminal matters (Wells, Malpass, Lindsay, Fisher, Turtle, & Fulero, 2000). This study regarding the effect of eyewitness testimony can be used to generate a discussion regarding the influence on juries of such testimony. Further, this study illustrates the value of using multiple group designs and serves as a point of departure for explaining post-hoc analyses.

The computer randomly assigns each participant to one of three different scenarios, in which the participant learns details of a crime. In the first scenario, the defendant provides testimony and argues he is not guilty. The second scenario is similar but also includes a statement that a witness saw the defendant commit the crime. In the third and final scenario, an attorney indicates that the witness was not wearing his glasses, thus his testimony may have been false. Loftus (1979) found that participants, acting in the role of juror would be more likely to convict someone on the basis of eyewitness testimony, even if that testimony were questioned.


Participants are in one of three treatment conditions (a) no eyewitness, (b) unrefuted eyewitness, and (c) discredited eyewitness. Participants in this study rate their belief about the defendant's guilt. The rating scale ranges from a 1 (definitely not guilty) to a 7 (definitely guilty).

Example Data Analyis

The most common analysis conducted with this study is a one-way ANOVA that explores whether there is a difference between the treatment conditions. Post-hoc analyses can illustrate which means differ.

Data Format and Download

Definitional information for each of the labeled columns appears in Figure 1.

Sample data image from the Be A Juror experiment
Figure 1

Column A provides the participant ID number that each person receives upon completion of the experiment. Column B specifies the participant's group: Condition 1) no eyewitness; Condition 2) unrefuted eyewitness; or Condition 3) discredited eyewitness. Column C reflects the dependent variable, the rating level of guilt, ranging from 1 (definitely not guilty) to 7 (definitely guilty). Data in columns D through H are self-explanatory: D, date of participation; E, time elapsed(seconds) during completion of study; F, participant's sex; G, participant's class affiliation (linked by an ID); and H, age of the participant.


Some fairly straightforward extensions could include varying the nature of the supposed witness, such as by age, and ethnicity. Such a manipulation could establish the credibility of people with differing characteristics. Another extension could involve varying the number of facts that the witness provides to see if a discredited witness gains credibility by reporting a large number of facts. This experiment lends itself to an analysis of variance with the treatment (three conditions) representing the independent varible and the level of guilt, the dependent variable. If results are consistent with those of the larger population, it is also possible to conduct a post hoc analyis comparing the two eyewitness conditions to the no eyewitness condition. Typically, the level of guilt will be reported higher whenever an eyewitness is present, even if the eyewitness reports innaccurate information.

Additional Reading

Using OPL to Illustrate the Concept of Memory


Loftus, E. F. (1974). Reconstructing memory: The incredible eyewitness. Psychology Today 
	8 (7), 116-119.
Loftus, E. F. (1979). Eyewitness Testimony. Cambridge, MA: Harvard University Press.
Wells, G. L., Malpass, R. S., Lindsay, R. C., Fisher, R. P., Turtle, J. W., & Fulero, S. M. (2000). 
	From the lab to the police station: A successful application of eyewitness research. 
	American Psychologist, 55, 581-598.