"When humans are looking for something, we know that they are more likely to miss that something if it is rare," said author Jeremy M. Wolfe, PhD, of the Visual Attention Lab, Brigham & Women's Hospital and Harvard Medical School. "This is the low prevalence effect and can be summed up with the one-liner, 'If you don't find it often, you often don't find it.'"
In the paper, Prevalence effects in newly trained airport checkpoint screeners: Trained observers miss rare targets, too, vision scientists report findings based on experiments with 125 newly trained transportation security officers (TSOs). The team of researchers found that the TSOs were better at detecting targets that appeared frequently as opposed to rarely.
Following their training, the TSOs participated in the experiments as part of their final evaluation. The TSOs were asked to search for images such as guns, knives and bombs in x-ray images of carry-on luggage. They searched for targets in five different sets of bags. The first three sets included targets that appeared at a low frequency, the fourth set had targets appear at a high frequency with feedback to let the observers know if they had been right or wrong and the fifth set of bags had targets appear at a low frequency again.
The results showed that in addition to being better at detecting targets that appeared frequently, the TSOs performed better when searching the set of bags where the target appeared rarely following the set where the potential threat appeared frequently with feedback activity. The authors suggest this provides some evidence that the short burst of high frequency with feedback may alleviate the prevalence effect and that such intervention might work for TSOs in the field as well. "However, much more research with TSOs and in the lab is needed before we have a good idea of how to minimize errors," said Wolfe.
Wolfe and his colleague have found prevalence effects under a wide range of laboratory conditions and view it as a robust part of the human search engine. For example, the research team found similar effects with radiologists performing breast cancer screenings. "It is important to show that professionals have to make do with the same 'search engine' as everyone else," said Wolfe, "They use it extremely well to perform a very difficult search task, but the fundamental limitations that constrain us amateurs constrain the professionals as well."