 Let \(\alpha (D_i)\) be a measure the strength of the probabilistic association between \(T\) and \(S\) in population  \(D_i\).[1]  By convention, \(\alpha (D_i) = 0\) corresponds to no association between the variables, \(\alpha (D_i) \gt 0\) indicates a positive association, and \(\alpha (D_i) &lt; 0\) a negative one. This can best be translated into the condition 