Usually, you want to control variables within a study. However, you also want to obtain a representative population to the population that you're measuring. How can one even strike a balance between the two? Is it by splitting the population into specific groups with the specific RELEVANT control variables, as you specified in this link (Is controlling variables important in deducing a statistically significant correlation?), and then observe the relevant outcomes?
I think that representation is also confusing because because I think to myself just because certain people live within a neighborhood, is there necessarily a link between them? There are probably certain aspects of representation which we have not been able to study yet.