The prevalence effect is the tendency for people to miss rare things they’re looking for:
In one experiment, Wolfe took 20 X-rayed images of luggage stuffed with guns and knives, and mixed those images into stacks of images of X-rayed luggage that didn’t have guns and knives.“If you stick those 20 bags into a stack of 40 bags, so on average there’s a gun and knife in 50 percent of the bags,” Wolfe says, “people missed about 7 percent of the bags.”
But when he took the exact same 20 bags and stuck them in a stack of 2,000 bags so that the targets showed up only 2 percent of the time, people got significantly worse. “All of a sudden, people were missing about 30 percent of the bags,” Wolfe says.