Chunk #41 — Methods and results — Testing for significance of gene-environment interaction effects using the reparameterized equation — Extensions of the equation
We have delineated models to test for interaction using a three category (G) variable, as that is the most common situation in which the problems delineated here would arise in the context of testing for gene-environment interaction. However, we note that the problems described here associated with using cross-product terms to test for interaction are not specific to tests of gene-environment interaction. They will exist when testing for interaction with any variable that has three or more categories. For a four or more category interaction variable it is possible to use powers of that variable (for n category variable regression one would include powers of that variable up to n − 1 and also interactions/multiplications of all powers with the other interacting variable). For example, the extension of the model to a four category variable (using G and E as the interacting variables for ease of comparison to the previous equations) would be P∼G+G2+G3+E+G×E+G2×E+G3×E.
