Additive and Multiplicative Models

Additive and multiplicative models are two alternative approaches to modeling effect of risk factors on disease. In additive models, risk—or disease incidence—changes by some fixed amount when a risk factor is present. The statement ‘If you take up X, you will increase your risk of Y by 10%’ is an example of an additive model of risk. In contrast, multiplicative models represent the changes in risk as a proportion of the baseline risk. The statement ‘If you take up X, you will double your risk of Y’ is an example of a multiplicative model of disease risk.

The distinction between additive and multiplicative models becomes especially important when considering the effect of multiple risk factors. For instance, consider the situation described in Table 1.