 Referring to the pioneering work of the statistician George U. Yule (1903: 132–134), Mittal (1991) calls this Yule’s Association Paradox (YAP). It is typical of spurious correlations between variables with a common cause, that is, variables that are dependent unconditionally (\(\alpha(D) \ne 0\)) but independent given the values of the common cause (\(\alpha(D_i) = 0\)). For example, sleeping in one’s clothes is correlated with having a headache the next morning. However, once we stratify the data according to the levels of alcohol intake on the previous night, the association vanishes: given the same level of drunkenness, people who undress before going to bed will have the same headache, ceteris paribus, as those who kept their clothes on.