We consider how to represent sigmoid-type regression relationships in a practical and parsimonious way. A pure sigmoid relationship has an asymptote at both ends of the range of a continuous covariate. Curves with a single asymptote are also important in practice. Many smoothers, such as...

There is increasing interest in the medical world in the possibility of tailoring treatment to the individual patient. Statistically, the relevant task is to identify interactions between covariates and treatments, such that the patient’s value of a given covariate influences how strongly (or...

Assessing the instability of a multivariable model is important but is rarely done in practice. Model instability occurs when selected predictors--and for multivariable fractional polynomial modeling, selected functions of continuous predictors--are sensitive to small changes in the data....