Additive model

A model in which the combined effect of several independent factors is the sum of the isolated effects of each factor.

Note: For example, if a factor X increases a risk by a in the absence of Y, and if a second factor Y increases the risk by b in the absence of X, the combined effect of the two factors is a + b.

(Related concept: multiplicative model)

Reference

http://htaglossary.net/additive-model