We extended the re-parameterization of the regression model to scenarios in which one of the interacting predictors is a categorical, or grouping, variable. If just two groups are present (e.g., low-risk vs. high-risk), only a single cross-over point is possible. If a ternary, or three-class, categorization into groups is used, alternate models can test whether a single cross-over point holds for all three groups or whether such a restriction should be rejected.
