How NOT to Bias Your Heterogeneous Treatment Effect Results

I can't stand it when people write about how what's typically done in the literature is wrong without providing a solution. Well, this is not one of those times.

Often we want to know how the effect of a treatment differs depending on people's outcome in the absence of the treatment. For example, do smaller class sizes help improve tests scores of the kids who typically score well or those that don't do so well?

The way you may think to answer such a question might lead to biased results, but don't worry, there's a way to fix it. Read here for a summary and here for the actual paper.