 Reichenbach used the apparatus of screening off to address the problem of spurious correlations. In our example, while a drop in the column of mercury (A) raises the probability of a storm (E) overall, it does not raise the probability of a storm when we further condition on the atmospheric pressure. That is, if A and E are spuriously correlated, then A will be screened off from E by a common cause. More specifically, suppose that \(C_t\) and \(E_{t'}\) are events that occur at times t and \(t'\) respectively. Then