The ultra-rare-item effect: visual search for exceedingly rare items is highly susceptible to error.

Accuracy is paramount in radiology and security screening, yet many factors undermine success. Target prevalence is a particularly worrisome factor, as targets are rarely present (e.g., the cancer rate in mammography is ~0.5%), and low target prevalence has been linked to increased search errors. More troubling is the fact that specific target types can have extraordinarily low frequency rates (e.g., architectural distortions in mammography-a specific marker of potential cancer-appear in fewer than 0.05% of cases). By assessing search performance across millions of trials from the Airport Scanner smartphone application, we demonstrated that the detection of ultra-rare items was disturbingly poor. A logarithmic relationship between target detection and target frequency (adjusted R (2) = .92) revealed that ultra-rare items had catastrophically low detection rates relative to targets with higher frequencies. Extraordinarily low search performance for these extraordinarily rare targets-what we term the ultra-rare-item effect-is troubling given that radiological and security-screening searches are primarily ultra-rare-item searches.