I’m mainly posting this because I’m getting tired of explaining it repeatedly! There are plenty of other better written articles about this topic but they don’t make the combination of points that I would like to make. This post is about correlation, and what you can and can’t use it for. More generally it is about being careful about drawing conclusions from data.
It is very easy to jump to conclusions when we see changes in the world around us. For example, you might be looking at the success of a particular vaccine in protecting populations from a particular disease. Let’s say that in the countries where the population is given this vaccine, the disease levels are lower.