How to examine external validity within an experiment

A fundamental concern for researchers who analyze and design experiments is that the estimate obtained from the experiment might not be externally valid for other policies of interest. Researchers often attempt to assess external validity by comparing data from an experiment to external data. In this paper, I discuss approaches from the treatment effects literature that researchers can use to begin the examination of external validity internally, within the data from a single experiment. I focus on presenting the approaches simply using stylized examples.