Sampling addresses variance, not confounding factors. If the sample is not representative of the broader population, then in no way will any amount of sampling

Sampling addresses variance, not confounding factors. If the sample is not representative of the broader population, then in no way will any amount of sampling correct that error. Observational studies often involve quite large samples, but they remain unreliable because it is extremely difficult to address all possible confounding factors. Experimental intervention is the gold standard for a reason, and absent the ethical or practical impossibility of experimentation, should be used here.