Correlation and causation

Science is often about measuring relationships between two or more factors. For example, scientists might want to know whether drinking large volumes of cola leads to tooth decay, or they might want to find out whether jumping on a trampoline causes joint problems.

• Correlation is when two factors (or variables) are related, but one does not necessarily cause the other
• Causation is when one factor (or variable) causes another

In the trampolining example, a study may reveal that people who spend a lot of time jumping on trampolines are more likely to develop joint problems, in which case it can be tempting to conclude that trampoline jumping causes joint problems. However, it might also be the case that the trampoline jumpers in the study were also long distance runners. Therefore, it is possible to say that there is a correlation between trampoline jumping and joint problems, but we do not know for sure whether trampoline jumping is the cause of the joint problems.

It is often easy to find evidence of a correlation between two things, but difficult to find evidence that one actually causes the other. When changes in one variable cause another variable to change, this is described as a causal relationship. The most important thing to understand is that correlation is not the same as causation – sometimes two things can share a relationship without one causing the other. For example, the more fire engines are called to a fire, the more damage the fire is likely to do. In this case, the damage is not a result of more fire engines being called. In fact, both variables (the number of fire engines and the amount of damage done) are caused by the size of the fire.

Even if there is a causal relationship between variables, it can be difficult to tell the direction of the relationship – which variable causes the other to change? For example, there might be a correlation between people’s mood and their physical health, but it is not obvious which variable influences the other – do good moods improve physical health, or does good physical health improve people’s moods?

Some types of research can give us evidence of causal relationships between two things, while other types can only help us to find correlations. For example, randomised controlled trials can provide good evidence of causal relationships, while cross-sectional studies such as a one-off surveys cannot.

When reading health research, it is important to remember the difference between correlation and causation, and question which, if either, of these the research is evidence of.