The Art of Thinking Clearly: Better Thinking, Better Decisions

See also Motivation Crowding (ch. 56); Reciprocity (ch. 6); Overconfidence Effect (ch.
15); Motivation Crowding (ch. 56)

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19
THE DUBIOUS EFFICACY OF DOCTORS, CONSULTANTS AND
PSYCHOTHERAPISTS
Regression to Mean
His back pain was sometimes better, sometimes worse. There were days when
he felt like he could move mountains, and those when he could barely move.
When that was the case – fortunately it happened only rarely – his wife would
drive him to the chiropractor. The next day he would feel much more mobile and
would recommend the therapist to everyone.
Another man, younger and with a respectable golf handicap of 12, gushed in a
similar fashion about his golf instructor. Whenever he played miserably, he
booked an hour with the pro, and lo and behold, in the next game he fared much
better.
A third man, an investment adviser at a major bank, invented a sort of ‘rain
dance’, which he performed in the restroom every time his stocks had performed
extremely badly. As absurd as it seemed, he felt compelled to do it: and things
always improved afterward.
What links the three men is a fallacy: the 
regression-to-mean
delusion.
Suppose your region is experiencing a record period of cold weather. In all
probability, the temperature will rise in the next few days, back toward the monthly
average. The same goes for extreme heat, drought or rain. Weather fluctuates
around a mean. The same is true for chronic pain, golf handicaps, stock market
performance, luck in love, subjective happiness and test scores. In short, the
crippling back pain would most likely have improved without a chiropractor. The
handicap would have returned to 12 without additional lessons. And the
performance of the investment adviser would also have shifted back toward the
market average – with or without the restroom dance.
Extreme performances are interspersed with less extreme ones. The most
successful stock picks from the past three years are hardly going to be the most
successful stocks in the coming three years. Knowing this, you can appreciate
why some athletes would rather not make it on to the front pages of the

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newspapers: subconsciously they know that the next time they race, they
probably won’t achieve the same top result – which has nothing to do with the
media attention, but is to do with natural variations in performance.
Or, take the example of a division manager who wants to improve employee
morale by sending the least motivated 3% of the workforce on a course. The
result? The next time he looks at motivation levels, the same people will not make
up the bottom few – there will be others. Was the course worth it? Hard to say,
since the group’s motivation levels would probably have returned to their
personal norms even without the training. The situation is similar with patients
who are hospitalised for depression. They usually leave the clinic feeling a little
better. It is quite possible, however, that the stay contributed absolutely nothing.
Another example: in Boston, the lowest-performing schools were entered into a
complex support programme. The following year, the schools had moved up in
the rankings, an improvement that the authorities attributed to the programme
rather than to natural 
regression to mean
.
Ignoring 
regression to mean
can have destructive consequences, such as
teachers (or managers) concluding that the stick is better than the carrot. For
example, following a test the highest performing students are praised, and the
lowest are castigated. In the next exam, other students will probably – purely
coincidentally – achieve the highest and lowest scores. Thus, the teacher
concludes that reproach helps and praise hinders. A fallacy that keeps on giving.
In conclusion: when you hear stories such as: ‘I was sick, went to the doctor,
and got better a few days later’ or ‘the company had a bad year, so we got a
consultant in and now the results are back to normal’, look out for our old friend,
the 
regression-to-mean
error.