He has made contributions to survival analysis, semiparametric statistics, multiple testing, and causal inference.

These may be defined as pieces of data that provide information about context and mechanisms, and that offer distinctive leverage in making causal inferences.

Conducting field experiments allows researchers to make causal inferences from their results, and therefore increases external validity.

We give it a special status in terms of our causal inferences.

On their own, quasi-experimental designs do not allow one to make definitive causal inferences; however, they provide necessary and valuable information that can not be obtained by experimental methods alone.