Answer: Restriction of Range.
Explanation: In most research studies such as statistical analysis to delineate the correlation or relationship between variables, choosing subjects to be used in the experiment remains of paramount importance. Once the chosen sample to be used in the experiment isn't representative, then we already have a failed experiment.
Dr. Steven's research sample choice exhibits a RESTRICTION OF RANGE PROBLEM whereby sample does not cut across the entire range or category of variables required to establish a meaningful relationship, hence the sampling population has been shortened or condensed.