In statistics and econometrics, we usually solve this confounder problem by controlling for C, i. e. by holding C fixed. This means that we actually look at different populations – those in which C occurs in every case, and those in which C doesn’t occur at all. This means that knowing the value of A does not influence the probability of C [P(C|A) = P(C)]. So if there then still exist a correlation between A and B in either of these populations, there has to be some other cause operating. But if all other possible causes have been controlled for too, and there is still a correlation between A and B, we may safely conclude that A is a cause of B, since by controlling for all other possible causes, the correlation between the putative cause A and all the other possible causes (D, E,. F …) is broken. This is, of course, a very demanding prerequisite, since we may never actually be sure to have identified all putative causes. Even in scientific experiments may the number of uncontrolled causes be innumerable. Since nothing less will do, we do all understand how hard it is to actually get from correlation to causality. This also means that only relying on statistics or econometrics is not enough to deduce causes from correlations. Some people think that randomization may solve the empirical problem. By randomizing we are getting different populations that are homogeneous in regards to all variables except the one we think is a genuine cause. In that way, we are supposed being able not having to actually know what all these other factors are. If you succeed in performing an ideal randomization with different treatment groups and control groups that is attainable. But — it presupposes that you really have been able to establish — and not just assumed — that the probability of all other causes but the putative (A) have the same probability distribution in the treatment and control groups, and that the probability of assignment to treatment or control groups are independent of all other possible causal variables. - larspsyll.wordpress.com