In medicine, it is essentially impossible to control for all possible factors. Case in point, ethnicity is not biologically realized either; it's a social tool we use to characterize broad swathes of phenotypic [and sociocultural] differences that are more likely (but not guaranteed) to occur in certain populations. But the example provided of indigenous arctic people is itself imprecise. You can't control for that, not without genetic testing - and even then, that presupposes we've characterized the confounding factors genetically, and that the confounding factors are indeed genetic in orgin at all.
Put another way, the population is simply too variable to attempt to eliminate all confounding factors. We can, at best, eliminate some of the ones we know about, and acknowledge the ones we can't.
Put another way, the population is simply too variable to attempt to eliminate all confounding factors. We can, at best, eliminate some of the ones we know about, and acknowledge the ones we can't.