As described earlier, a set of follow-up analyses were conducted to determine whether the linear additivity assumption could be confirmed regarding the robust risk factors examined in these models. A total of six 2-way interactions were created from the alcohol-related covariates of LR, family history, T1 typical quantity, and age of alcohol onset. Two interactions yielded a significant LRT when included in the full model from Table 2: family history by T1 quantity (χ2 = 4.146, df=1, p=.04; b=.30, p=.03) and age of onset × T1 quantity (χ2 = 4.464, df=1, p=.03; b=.07, p=.01). Thus, allowing for these interactions improved overall model fit and there was evidence to reject the linear additivity assumption for these covariate effects.
