Internal validation of models with several interventions

In cost-effectiveness analyses, models are used typically to synthesize the best available data and/or extrapolate beyond clinical trial data. Ideally, models should be validated both internally and externally. The purpose of this paper is to suggest a test for internal validation of models where several interventions for the same clinical indication are compared. To the best of our knowledge, such a specific test does not yet exist. There are four versions of the test, which consider the relationship between incremental downstream costs and effects in the case of a single or several endpoints. We apply two versions of the validation test to published cost-effectiveness analyses of physical activity programs and demonstrate internal validity of the model in one study and lack of internal validity of the model in the other study.