The most widely used smoothing function is of course a linear approximation, which served as the workhorse of most quantitative work in the social and behavioral sciences in the twentieth century. We have seen how Sewall Wright embedded his path diagrams into the context of linear equations, and we noted there one computational advantage of this embedding: every causal effect can be represented by a single number (the path coefficient). A second and no less important advantage of linear approximations is the astonishing simplicity of computing the adjustment formula.
