Answer:
quasi-experimental
Explanation:
Quasi-experimental approach is an empirical study used to adjudge the impact of a cause on target population without random assignment.
Quasi-experiments are subject to concerns regarding internal validity, this is true in the case Drs. Goran and Lieberman who accessed the differences in tolerance of pain (pain threshold) and gender between men and women because the treatment and control groups may not be comparable at baseline.