Let’s take a look at some of the surrogate definitions of confounding. These fall into two main categories, declarative and procedural. A typical (and wrong) declarative definition would be “A confounder is any variable that is correlated with both X and Y.” On the other hand, a procedural definition would attempt to characterize a confounder in terms of a statistical test. This appeals to statisticians, who love any test that can be performed on the data directly without appealing to a model.
