Several more complex models allowed non-additive associations to exist among the predictor disorders. One of these added to the additive multivariate model a series of dummy variables for number of prior lifetime disorders beginning with two. These dummy variables represent gross interaction terms that require the coefficient to be the same for all combinations of disorders of a given number (e.g., all pairs of disorders, all sets of three disorders). A somewhat more complex model weighted the disorders in each of these sets to have differential slopes proportional to the main effects of the disorders. Another specification allowed for interactions between a continuous measure of number of prior lifetime disorders and each type of prior lifetime disorders. More detailed information about these models are presented in a previous report in this journal (Alonso et al., 2011). The best-fitting model among these alternatives was selected using the Akaike Information Criterion (AIC) (Burnham & Anderson, 2002). Survival coefficients and standard errors were exponentiated to produce odds-ratios (ORs) and 95% confidence intervals.
