The Illusion of Validity

Saturday, January 7th, 2012

When Daniel Kahneman was a young lieutenant in the Israeli Defense Forces, he found that a simple questionnaire-driven statistical model did a better job of predicting which recruits could perform which jobs than the thorough interviews and “expert” opinions the IDF was relying on. But the illusion of validity was strong — which Freeman Dyson also found while performing operational research in Britain during World War II:

An episode from my own past is curiously similar to Kahneman’s experience in the Israeli army. I was a statistician before I became a scientist. At the age of twenty I was doing statistical analysis of the operations of the British Bomber Command in World War II. The command was then seven years old, like the State of Israel in 1955. All its institutions were under construction. It consisted of six bomber groups that were evolving toward operational autonomy. Air Vice Marshal Sir Ralph Cochrane was the commander of 5 Group, the most independent and the most effective of the groups. Our bombers were then taking heavy losses, the main cause of loss being the German night fighters.

Cochrane said the bombers were too slow, and the reason they were too slow was that they carried heavy gun turrets that increased their aerodynamic drag and lowered their operational ceiling. Because the bombers flew at night, they were normally painted black. Being a flamboyant character, Cochrane announced that he would like to take a Lancaster bomber, rip out the gun turrets and all the associated dead weight, ground the two gunners, and paint the whole thing white. Then he would fly it over Germany, and fly so high and so fast that nobody could shoot him down. Our commander in chief did not approve of this suggestion, and the white Lancaster never flew.

The reason why our commander in chief was unwilling to rip out gun turrets, even on an experimental basis, was that he was blinded by the illusion of validity. This was ten years before Kahneman discovered it and gave it its name, but the illusion of validity was already doing its deadly work. All of us at Bomber Command shared the illusion. We saw every bomber crew as a tightly knit team of seven, with the gunners playing an essential role defending their comrades against fighter attack, while the pilot flew an irregular corkscrew to defend them against flak. An essential part of the illusion was the belief that the team learned by experience. As they became more skillful and more closely bonded, their chances of survival would improve.

When I was collecting the data in the spring of 1944, the chance of a crew reaching the end of a thirty-operation tour was about 25 percent. The illusion that experience would help them to survive was essential to their morale. After all, they could see in every squadron a few revered and experienced old-timer crews who had completed one tour and had volunteered to return for a second tour. It was obvious to everyone that the old-timers survived because they were more skillful. Nobody wanted to believe that the old-timers survived only because they were lucky.

At the time Cochrane made his suggestion of flying the white Lancaster, I had the job of examining the statistics of bomber losses. I did a careful analysis of the correlation between the experience of the crews and their loss rates, subdividing the data into many small packages so as to eliminate effects of weather and geography. My results were as conclusive as those of Kahneman. There was no effect of experience on loss rate. So far as I could tell, whether a crew lived or died was purely a matter of chance. Their belief in the life-saving effect of experience was an illusion.

The demonstration that experience had no effect on losses should have given powerful support to Cochrane’s idea of ripping out the gun turrets. But nothing of the kind happened. As Kahneman found out later, the illusion of validity does not disappear just because facts prove it to be false. Everyone at Bomber Command, from the commander in chief to the flying crews, continued to believe in the illusion. The crews continued to die, experienced and inexperienced alike, until Germany was overrun and the war finally ended.


  1. M.L. Wilson says:

    Also worth noting, the British bomber with the lowest loss rates (by an extremely wide margin, too) over occupied Europe and Germany was the Mosquito, which had no defensive armament and was was made of wood!

    Cochrane was almost certainly correct.

    I recall a similar study by the Americans which found that evasive maneuvers were counterproductive for bombers caught in flak. The flak was essentially random enough that trying to dodge it just meant that the bomber would spend a longer time getting shot at and thereby increase the chances of getting hit.

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