Association
When two variables are related, we say that there is association between them. Consider the height, X, and weight, Y, of a sample of school children. Tall children tend to be heavier, so high values of X are associated with high values of Y. The correlation coefficient describes the amount of linear association between two such numerical variables.
Causal and non-causal relationships
Association between two variables may be causal — one variable directly affects the other. Variables may however be associated without the relationship being causal.
If two variables are causally related, it is possible to conclude that changes to the explanatory variable, X, will have a direct impact on Y, but if the variables are not causally related, it is impossible to tell whether changes to one variable, X, will result in changes to the other variable, Y.
Churches and cancer deaths
Researchers usually want to detect causal relationships |