Chance guessing in a forced-choice recognition task and the detection of malingering.

Guessing occurs on forced-choice (FC) tests for which responses cannot be based on relevant knowledge. Its importance is in inverse relation to the level of knowledge being measured, so that it becomes an increasing component of test scores as the level of knowledge decreases. It is also used as a benchmark to detect simulated impairment. This investigation examined the role of guessing in a 2-alternative FC face recognition test. Chance groups shown only the test items were asked to guess which were the targets, to measure the variation in scores likely to be found with pure guessing. Controls performed normally, and two simulation groups tried to fake amnesia. Results suggested that simple guessing in the chance group produced variable scores that overlapped both low genuine and "malingering" performance. Low control scores were hidden by the guessing "chance bonus," which ameliorated the apparent level of decline in memory. Simulators told of their role at the outset (before presentation of the target items) produced more convincing "amnesic" scores than those told only just before the test, who produced the expected below-chance level of score. It is suggested that guessing variability should be taken into account in interpreting FC scores.