In statistical terminology it would be said that a positive correlation has been found to exist between these two things.
What sounds like proof of an ancient myth is actually something far more valuable. It is an easily remembered reminder of a useful truth: an association between two factors is not proof that one has caused the other.
In the instance of the storks and the babies, it is not too hard to find a third factor that may be responsible for the other two. Big houses attract big, and potentially big, families; and big houses have more chimney pots on which storks may nest.
But flaws in assumptions of causality are not always so easy to spot, especially when the relationship seems to make a lot of sense or when it pleases a popular prejudice.
Somebody once went to a good deal of trouble to find out if cigarette smokers make lower college grades than non-smokers. It turned out that they did. This pleased a good many people and they have been making much of it ever since. The road to good grades, it would appear, lies in giving up smoking; and, to carry the conclusion one reasonable step further, smoking makes dull minds.
The particular study was, I believe, properly done: sample big enough and honestly and carefully chosen, correlation having a high significance, and so on.
The fallacy is an ancient one which, however, has a powerful tendency to crop up in statistical material, where it is disguised by a welter of impressive figures. It is the one that says that if B follows A, then A has caused B. An unwarranted assumption is being made that since smoking and low grades go together, smoking causes low grades. Couldn’t it just as well be the other way around? Perhaps low marks drive students not to drink but to tobacco. When it comes right down to it, this conclusion is about as likely as the other and just as well supported by the evidence. But it is not nearly so satisfactory to propagandists.
It seems a good deal more probable, however, that neither of these things has produced the other, but both are a product of some third factor. Can it be that the sociable sort of fellow who takes his books less than seriously is also likely to smoke more? Or is there a clue in the fact that somebody once established a correlation between extroversion and low grades – a closer relationship apparently than the one between grades and intelligence? Maybe extroverts smoke more than introverts. The point is that when there are many reasonable explanations you are hardly entitled to pick one that suits your taste and insist on it. But many people do.
uit: How to lie with statistics - Darrell Huff
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