Re: [SystemSafety] Fwd: Contextualizing & Confirmation Bias

From: Denis Besnard < >
Date: Thu, 06 Feb 2014 20:53:23 +0100

Dear all,

I am a silent follower of this list and I apologise for having lost track of where the the confirmation bais came from in this thread. Anyway, I thought I would humbly inject the following.

Nancy wrote:
> It would be helpful if you would provide references if you are going to
> provide definitions that differ greatly from the widely accepted.

As far as I know, one reference goes back to the Wason task. He (she?) designed a task where subjects had to decide which of a set of 4 cards would have to be turned over to test whether a logical statement holds. You can find a recap of this seminal experiment at

There is controversy over the interpretation of the results but all in all, it is taken as the first demonstration that people, when reasoning, have a tendency to select cases that confirm (as opposed to reject) their hypotheses.

An example of the above I saw with my own eyes was that of a researcher who came to Newcastle University (UK) for a talk (1). He plugged the VGA cable into his Mac laptop with a adapter. After a while, the signal was lost and the picture on the screen when black. He moved the laptop around a bit, touched the adapter and the picture came back on. This happened twice in a row and he treated the problem in the same way each time. At that point, it was obvious that his adapter was a bit loose. Now, had he wanted to know for sure if the adapter was at fault, a powerful test would have been that of assuming that the adapter is OK and that another cause is at play. He did not even have to try. The case came uninvited. The screen went black once more but this time, for some reason, this person accidentally touched the touchpad and the picture came back on right away. The laptop monitor was simply going to sleep.

What I get from the above is that testing a hypothesis on the basis of information that would confirm a suspected fault can lead to flawed conclusions. A complementary test would be one where one accepts the hypothesis on the basis of information that fails to reject the suspected cause.

An industrial example of all this happened in the cockpit of the B737 that crashed at Kegworth in 1989. The crew throttled back one engine that was suspected to vibrate. Very shortly after that, the vibration level decreased on that engine, leading the crew to believe that they had diagnosed the problem right. They had just tested a hypothesis of fault about this engine and accepted it on the basis of confirming evidence. The counterpart of the reasoning would have been to try to find a test that would reject this hypothesis. This could have been e.g. increasing the thrust on the same engine and see what would happen. Their prediction would have been that vibrations would resume but chances are they would NOT have. Indeed, the AAIB report established that the engine they had throttled back was the healthy one. The drop in vibrations only came from the erratic behaviour of the faulty engine, the only one they had to finish their fatal flight with.

The whole process of overestimating the weight of confirming evidence is a flaw that I have seen in many reasoning tasks. Troubleshooting is a wonderful source in this respect and I have spent some time studying it. I suppose other classes of reasoning such as experimental plan design might be sensitive to the confirmation bias although I cannot recollect any right now.


(1) It was in room 911, Comp Science Dept in Claremont tower; hi folks!

Denis Besnard
Co-Director of the post-Master's degree in Industrial Safety: FHOMSI
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Received on Thu Feb 06 2014 - 20:53:36 CET

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