Chunk #28 — Methods and results — A reparameterization of the equation to address the problem

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Testing for measured gene-environment interaction: problems with the use of cross-product terms and a regression model reparameterization solution.

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In Fig. 2c the simulated data for each genotypic group do not cross at the same point (note that this also means that the slopes are not ordered in an additive fashion). Again, although all three real lines do not cross at one point, the four parameter model shows all three lines as crossing. It yields a highly significant interaction term with p < 10−16, but the predicted regression lines are a very poor representation of the data. The six parameter model accurately reflects the interaction.