Clinical Trials

❘❙❚■ Chapter 27 | InteractionFigure 1. Plots demonstrating quantitative and qualitative interaction.(A = no interaction; B and C = quantitative interaction; D = qualitative interaction).Mean change in SBP (mm Hg)10–1–2–3–4–5ASmokerNonsmokerSmoking statusPlaceboDrugMean change in SBP (mm Hg)10–1–2–3–4–5BSmokerNonsmokerSmoking statusPlaceboDrugMean change in SBP (mm Hg)10–1–2–3–4–5CSmokerNonsmokerSmoking statusPlaceboDrugMean change in SBP (mm Hg)10–1–2–3–4–5DSmokerNonsmokerSmoking statusPlaceboDrugSBP = systolic blood presssure.two lines cross each other, a qualitative interaction of this kind is also sometimescalled a crossover interaction.Evaluation of an interaction effectTo formally evaluate whether there is an interaction effect between two variablesand the magnitude of that effect, statistical tests must be performed. There area number of statistical methods for the evaluation of interaction effects [1–3].The most commonly used method is to use a regression model. Which regressionmodel to choose depends on the type of data to be analyzed. For example, linearregression is most suitable for a continuous outcome measure (eg, SBP),while a logistic regression is preferred for a binary outcome (eg, death) (seeChapter 24).308