Prevalence Effect

The prevalence effect is the finding that people are more likely to miss a target during a visual search when that target occurs rarely than when it occurs often, even when the rare target is just as important to catch. Studies have found that detection errors rise sharply as a target's frequency falls, from around 7 percent missed at 50 percent prevalence to about 16 percent missed at 10 percent prevalence and beyond 30 percent missed when a target appears less than 1 percent of the time, a pattern demonstrated in research by Jeremy Wolfe and colleagues using tasks modeled on airport baggage screening, where weapons are rare and therefore easy to overlook. The effect is understood as a shift in an observer's decision threshold: because a target is expected to be absent most of the time, the visual system becomes biased toward reporting not found, which is efficient on average but causes more misses precisely on the rare occasions the target is actually present.