Chunk #27 — Methods and results — A reparameterization of the equation to address the problem

Source

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

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Figure 2b illustrates the case when differences between slopes are different in the simulated data, but the other two constraints met. Because the four parameter model requires the slope differences to be the same between genotypic groups, the implied regression lines from the model will always be a poor fit to the actual shape of the data, as illustrated in the figure.