Correlation and causality

Two events that occur simultaneously or only slightly offset in time can, but do not have to be causally connected (causality). The simultaneous occurrence of two facts (correlation) can also always be pure coincidence or be based on a third (or even several) common but external cause(s). Confusing these two terms can lead to serious misjudgments, since one does not or only half-heartedly research an actual cause based on a simple correlation, which can, however, be pure coincidence. A conscientious scientist who discovers a correlation will first of all only take this as an opportunity to look for a possible causality (connection), but never assume a causality in advance, however tempting or desirable it may be depending on the situation.

An example of an external cause:

People in the Northern Hemisphere are often tall, have big feet and tend to be fair-skinned, blue-eyed and blond (correlation); the conclusion that body size is causal for light hair, or that blue eyes are causal for big feet seems rather absurd - it would be much more logical to look for an external factor, in this case common genetics, and analyse this as a potential influencing factor.

An example of pure coincidence:

This is best illustrated by the following graph, which relates the number of people drowned in a swimming pool to films in which Nicolas Cage appears. (Found at https://www.tylervigen.com/spurious-correlations, where you will find more amusing examples). It is very likely that the correlation is purely coincidental, a causality can probably be ruled out.