Is a relationship causal?

Investigators usually hope to find causal relationships between the variables that are recorded. If one variable causally affects the other, then adjusting the value of that variable will cause the other to change.

Causality can only be determined by reasoning about how the data were collected. The data values themselves contain no information that can help you to decide.

Lurking variables

Non-causal relationships between two variables usually result from the effect of further variables called lurking variables that are related to the variables under investigation. Causal relationships can only be deduced if it can be reasoned that lurking variables are not present.