We compared three survival models that place different constraints on how the hazard rate is modeled. Model 1 (M1) is unconstrained. This means that the hazard can be different in each time interval, and that the hazard is regressed on the ten SNPs. In this model, the SNPs have time interval-specific regression weights. This model is included as a lenient baseline model. Model 2 (M2) constrains the hazard to be proportional. This model permits the hazard to increase and/or decrease over time, however, covariate effects follow the proportionality constraint such that only one regression coefficient is estimated for each SNP. Model 3 (M3), finally, constrains the hazard to be constant over time. Again, there is only one regression coefficient estimated per covariate. The substantive meaning of the constant hazard is that for the observed time span (i.e. adolescence) the risk of onset does not change substantially. Although due to availability there might be a peak risk at a younger age for some substances, it might be possible that the risk of initiating use of other substances is fairly constant during adolescence.
