In college my statistics professor’s favorite expression was “correlation does not imply causation.” In case you’re not familiar with the phrase, I’ll borrow the explanation I learned in school:
When male college students wake up with a headache, a large percentage of the time they are still wearing their shoes. Therefore, sleeping with shoes on is correlated with waking up with a headache. However, it would be incorrect to conclude that sleeping with shoes on causes headaches. The more likely explanation is that both are caused by going to bed drunk. Correlation is not the same as cause.
While the example is vivid, it’s not particularly practical in distinguishing between correlation and cause. It’s unlikely anyone thought shoes caused the headaches. However, we tend to confuse correlation and cause frequently. In fact, most of the research studies I read are designed to test correlation even though the popular press reports it as cause.
For example, many studies have shown that eating breakfast is correlated with success in elementary school. This has turned into the popular wisdom that eating a good breakfast causes students to be better learners. However, this isn’t really the case; students who don’t eat breakfast are also more likely to be absent or tardy which in turns causes their poor performance. As it turns out, breakfast only helps undernourished children perform better.
As humans, we gravitate towards order and explanations of the unknown. This means we are constantly searching for causes, even if they don’t exist. Erik Hollnagel explains in his book ‘Barriers And Accident Prevention’:
"Whenever an accident happens there is a natural concern to find out in detail exactly what happened and to determine the causes of it. This trait of human nature is so strong that we try to find causes even when they do not exist, such as in the case of misleading or spurious correlations. One very good reason is that we have created a way of living that depends heavily on the use of technology, and that technological systems are built to function in a deterministic, hence reliable manner. A second reason is that our whole understanding of the world is based on the assumption of specific relations between causes and effects, as amply illustrated by the Laws of Physics. A third reason is that most humans find it very uncomfortable when they do not know what to expect, i.e., when things happen in an unpredictable manner. This creates a sense of being out of control, something that is never desirable since – from an evolutionary perspective – it means that the chances of survival are reduced."
So, be wary of your own biases. Two events occurring frequently together does not mean one caused the other, even if it seems to makes perfect sense. Especially if it involves headaches.
