Recent Posts

Perhaps you’ve seen a claim like this in an applied paper: “the estimated effect for Group A is statistically significant, but the estimated effect for Group B is not; this treatment helps As but not Bs.

At a recent seminar dinner the conversation drifted to causal inference, and I mentioned my dream of one day producing a Lady Gaga parody music video called “Bad Control”.1 A lively discussion of bad controls ensued, during which I offered one of my favorite examples: a good instrument is a bad control.

The result that I prefer to call Yule’s Rule, more commonly known as the “Frisch-Waugh-Lovell (FWL) theorem”, shows how to calculate the regression slope coefficient for one predictor by carrying out additional “auxiliary” regressions that adjust for all other predictors.

Suppose I run a simple linear regression of an outcome variable on a predictor variable. If I save the fitted values from this regression and then run a second regression of the outcome variable on the fitted values, what will I get?

Welcome to the first installment of the Econometrics Puzzler, a new series of shorter posts that will test and strengthen your econometric intuition. Here’s the format: I’ll pose a question that requires only introductory econometrics knowledge, but has an unexpected answer.