How to Establish Causation

It is often important to determine causation, or the existence of a cause and effect relationship between two variables. When we see an apparent relationship between two variables it is tempting to conclude that one caused the other. We do this often in everyday life. Sometimes this causes us to make mistakes as we reach false conclusions. In Statistics we will try to avoid some of the common pitfalls, and to recognize why they occur.

In the best of all worlds, when we want to identify cause and effect, we use a well-designed controlled experiment. This is the most direct and reliable way of correctly reaching a conclusion. So, for best results: use a controlled experiment. Sometimes, however, this cannot be achieved. If you cannot carry out an experiment, you must build a case to support your contention that changes in one variable cause changes in another.

Without an experiment, build a case: Look for a strong association. Look for a dose-dependence, i. e., increasing amounts of the alleged causative variable result in increased amounts of resulting variable. (e.g., We suspect that sugar on teeth causes cavities. If children eat large amounts of candy, we expect more cavities than among those who eat little candy, due to an increase in the amount of sugar on teeth.) Look for a consistent association. The alleged cause must precede the effect. It helps if the alleged cause is plausible. Sometimes, however, a lack of knowledge may prevent us from having the insight to see this.