the formula (Equation 7.1), which cannot be found in ordinary statistics textbooks. Here X stands for Smoking, Y stands for Cancer, Z stands for Tar, and U (which is conspicuously absent from the formula) stands for the unobservable variable, the Smoking Gene. P(Y | do(X)) = Σz P(Z = z, | X) Σx P(Y | X = x, Z = z) P(X = x)(7.1) Readers with an appetite for mathematics might find it interesting to compare this to
