Bias is good
November 12, 2013
How studying the history and philosophy of EBM helps
Evidence-Based Medicine (EBM) is about reducing bias. Therefore, as an Evidence-Based Medicine researcher, how can I say bias is good?
The answer ultimately depends on the difference between ‘cognitive bias’ (in your head) and study bias …I’ll try and explain with a simple example about you or someone you know.
How many people do you know (including yourself) who are scared to fly in an airplane? How many people do you know (including yourself) who are scared to drive in a car? You probably know more people who are scared to fly, as an estimated 40% of plane passengers report fear of flying. Yet, fear of flying often remains even when people are told that they are 73 times more likely to die in a car accident than in a plane accident.
The reason this irrational fear persists is the ‘availability heuristic,’ a bias that commonly occurs in our thinking.
Since plane crashes, when they occur, are reported widely, our brain tells us they are more likely even when we know the ‘facts’. We are therefore irrational and we can’t help it.
Availability bias can be put to good use if we remember to use dramatic memorable stories to illustrate why it is important to reduce study bias.
For example, a common study bias is that they ignore the fact that many people recover from disease without treatment (‘natural history’). Patients get a drug, get better, and we assume it was because of the drug. But the very same patient might have recovered without the drug (and saved money and time to boot). The fancy name for this fallacious reasoning is post hoc ergo procter hoc (‘after therefore because of’).
Abstract lectures about natural history, or the post hoc fallacy, are likely to go in one ear and out the other, perhaps generating a yawn on the way. On the other hand, if we tell the true story about Archie Cochrane’s experience in a German POW camp, people are much more likely to remember the power of natural history:
… my four years as a prisoner of war in German hands … educated me in two very different ways … I was usually the senior medical officer and for a considerable time the only officer and the only doctor… There were about 20,000 POWs in the camp, of whom a quarter were British. The diet was about 600 calories a day and we all had diarrhoea. In addition we had severe epidemics of typhoid, diphtheria, infections, jaundice, and sand-fly fever, with more than 300 cases of ‘pitting oedema above the knee’. To cope with this we had a ramshackle hospital, some aspirin, some antacid, and some skin antiseptic.
The only real asset were some devoted orderlies, mainly from the Friends’ Field Ambulance Unit. Under the best conditions one would have expected an appreciable mortality; there in the Dulag I expected hundreds to die of diphtheria alone in the absence of specific therapy. In point of fact there were only four deaths, of which three were due to gunshot wounds inflicted by the Germans. This excellent result had, of course, nothing to do with the therapy they received or my clinical skill. It demonstrated, on the other hand, very clearly the relative unimportance of therapy in comparison with the recuperative power of the human body. On one occasion, when I was the only doctor there, I asked the German Stabsarzt for more doctors to help me cope with these fantastic problems. He replied: ‘Nein! Aerzte sind ueberfluessig.’ (‘No! Doctors are superfluous.’) I was furious and even wrote a poem about it; later I wondered if he was wise or cruel; he was certainly right.
This amazing story illustrates the fact that many people get better without any treatment than an abstract lecture about natural history.
To hear about other fascinating examples from the history of EBM, which will make you a better informed patient, doctor, or policy maker, consider signing up to our course on the History and Philosophy of EBM.