In designing research study it is important to identify your target population (those who will participate in the study, usually the population that would most benefit from the outcome of the study). This is usually accomplished by identifying the group of individuals with the least amount of variables that will benefit the most from your results. The study question is a good place to start to identify the target population. In the example study, the target population are diabetics who will test a specially designed sock for diabetics. The two choices of validity to identify the target population are internal and external. The best target populations are the ones with minimal variables to address the question at hand. If the target population is to be a general one then external validity would be used to identify them. However, is a more select group is required an internal validity would need to be applied.

Truly your target population and method of validity depend on the purpose of your study. I would suggest that for these soaks it would be most beneficial to identify the diabetics that are most prone to excoriation as an internal validity criteria.