Prevalence Effect

The concept emerged from 1960s vigilance research, with psychologist Jane Mackworth noting in 1970 that signal detection decreases when background event rates increase. Green and Swets illustrated in their 1967 text Signal Detection Theory that reducing target prevalence shifts decision criteria more conservatively. Modern systematic research in the 2000s revealed that miss rates spike at low prevalence (1–2%) compared to high prevalence (50%), explained as a criterion shift rather than reduced sensitivity.