Rare objects harder to find: study
Screeners searching for weapons like guns or bombs are more prone to error when the incidence of such threats is small, say US researchers.
When people look for something rare - like a gun or knife or an explosive device hidden in a suitcase - they often have trouble spotting it, researchers have found.
The reverse is also true. When something is very common, people tend to see it everywhere they look, even when if it is not there, says Professor Jeremy Wolfe of Harvard Medical School, whose study appears in the journal Current Biology.
"It is clear that if you don't find it often, you often don't find it," he says.
That means that if you look for 20 guns in a stack of 40 bags, you'll find more of them than if you look for the same 20 guns in a stack of 2000 bags.
"We really want to understand why that is happening," says Wolfe.
For the current study, Wolfe and his colleagues worked with the US Department of Homeland Security's Transportation Security Laboratory. The department sent Wolfe's team images of empty suitcases and images of items typically found in them.
"We have software that basically packs bags. It takes an empty bag and throws some clothes in it and maybe some odds and ends - and maybe it throws a gun or a knife in there," Wolfe said. "We either had those show up 50% of the time or 2% of the time," he says.
Then they recruited 13 volunteers to look for guns or knives in the bags.
What they found was people were much more likely to spot a weapon when there were a lot of them tucked away in the luggage; but when they were rare, they had much more trouble finding them.
"If it shows up 50% of the time, in our particular experiments, they missed about 7% of the targets that were there. If it shows up on only 2% of the trials, they missed about 30% of the time," Wolfe said.
"Basically, you'll miss more of them in that big stack than in the little stack," he says.
Wolfe says understanding how humans find things is critical to tasks like baggage screening in airports because none of the scanners currently in use - or even the more advanced whole body scanners - can spot risks without the aid of humans.
The same is true for other screening tasks, Wolfe says, like mammography screening for breast cancer. "At this point, there is no computer that can look at an X-ray of a breast and say, 'I can tell you with absolute certainty that this is or is not bad.'"
Computer systems are quite good at pointing out trouble spots, but they often point out too many potential risks, making it harder for screeners to find the real threats.