Weapons Screening and Human Error

With all the understandable emphasis being placed on full body scanners and other detection technologies in the week since the failed Flight 253 bombing attempt, it’s easy to forget that the ultimate success or failure of airport remains the “humanfactor”, specifically the human screeners tasked with interpreting machine image scans.
A new report published last week in Current Biology, a Cell Press publication (no link available yet), examines the source of human error in screening for weapons and suggest some simple methods to help airport security personnel looking for weapons.
The study, led by Jeremy Wolfe of the Harvard Medical School in collaboration with the US Department of Homeland Security’s (DHS) Transportation Security Laboratory, studied the responses of 13 volunteers who looked for guns or knives in a software generated stream of images of empty suitcases filled with images of items typically found in them.
The study, with the academic title Varying Target Prevalence Reveals Two Dissociable Decision Criteria in Visual Search, ran two tests, one in which a gun or a knife showed up 50 percent of the time, another in which it appeared 2 percent of the time.
Its core finding: when people look for things that are rare, they aren’t all that good at finding them. And, conversely, when people look for something common, they will often think they see it even when it isn’t there.
“When targets are rare,” the study said, “observers shift response criteria, leading to elevated miss error rates”, while “observers also speed target-absent responses and may make more motor errors .”
“ This could be a speed/accuracy tradeoff,” the study suggests, “ with fast, frequent absent responses producing more miss errors.”
At the same time the very high target prevalence tests showed shifts response criteria in the opposite direction, leading to elevated false alarms in a simulated baggage search.
The false-alarm rate increased dramatically at the high-prevalence level, while miss errors dropped.
Explaining the results in a statement Wolfe said, "We know that if you don’t find it often, you often don’t find it. Rare stuff gets missed."
That means, Wolfe added, “ 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 2,000 bags.
Wolfe explained these error patterns as possibly “hard-wired” in the evolution of perceptual systems.
"It’s all terribly adaptive behavior for a beast in the world,” he said. “If you know berries are there, you keep looking until you find them. If they are never there, you don’t spend your time hunting."
It’s that adaptive inclination in nature that Wolfe believes can cause problems when people start looking for rare things, like guns in baggage.
Wolfe believes there may be ways to retrain airport screeners to reorient their searching skills.
The study proposes that error rates may be lowered by offering people in screening jobs simple retraining at the start of every shift.
“ If they spend a couple of minutes doing a simulated search for common weapons or tumors, they might then do a better job at really finding rare ones for the next 30 minutes or so,” Wolfe speculated.
The researchers plan to conduct further tests in actual airport environments, to determine whether the effects seen in the laboratory apply in real world high- stakes situations.

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