Effects of prevalence on visual search and decision making in fracture detection

Research concerning disease prevalence has inferred that indices of observer performance become, in part, a function of predetermined prevalence. The cause of this modified performance and decision-making is not fully understood, although the alteration of the criterion level of the observer may be a feature. Novice radiography students were randomly assigned to one of three digitised test banks of 72 wrist images. Test bank A, B and C represented a fracture prevalence of 50%, 83% and 22% respectively. Half of the observers from each group were made aware of the prevalence of their respective test bank. Observers recorded their decisions on an operator rating scale. Results showed significant differences in overall Az between the 50% and 83% prevalence sets (p=0.04) and the 50% and 22% prevalence sets (p=0.005). Knowledge of the prevalence influenced both sensitivity and specificity values at the 83% prevalence level (p=0.03 and 0.02) but not at the lower levels. For test bank A sensitivity was 87%; specificity 53%; Az 0.80, test bank B sensitivity 81%; specificity 48%; Az 0.71 and test bank C sensitivity 85%; specificity 43%; Az 0.68. Analysis of the eye movement patterns of observers under conditions of varying prevalence is in progress.