Effects of item repetitions and variations on the efficiency of the guilty knowledge test


  • This research was supported by a grant from the Sturman Center for Human Development at the Department of Psychology. The Hebrew University of Jerusalem.

  • We thank Ygaal Atali. Ariel Goler. Sharon Liberty, Eshkol Refaeli, and Michal Tur-Kaspa for their help with the data collection of Experiment 1. In addition we are grateful to Maya Bar-Hillel. Peter Rosenfeld, and an anonymous reviewer for their helpful comments on an earlier version of this manuscript. Finally. we thank Yosi Rinot for his advice regarding the statistical analyses of the ROC data.

Dr Eitan Elaad. Division of Identification and Forensic Science. Israel National Police H.Q., Jerusalem 91906, Israel.


Two mock-crime experiments were designed to examine the effects of question repetition and variation on the efficiency of the guilty knowledge test. In the first experiment, two factors (single vs. multiple questions, and high vs. low motivation) were manipulated and in the second experiment, which was an analog study conducted in a police laboratory using field equipment, only the first factor was examined. Similar detection efficiencies were obtained in both experiments when a single question was repeated 12 times, and when each of four different questions was repeated 3 times. The results also showed that accumulating information across repetitions and across physiological measures tended to increase detection efficiency and reduce false positive errors. The results of Experiment 1 suggest that motivation to avoid detection may be associated with better detection efficiency.