Interrupted time series analysis is a quasi-experimental design that can evaluate an intervention effect, using longitudinal data. The advantages, disadvantages, and underlying assumptions of various modelling approaches are discussed using published examples

English family practices attained high levels of achievement in the first year of the new pay-for-performance contract. A small number of practices appear to have achieved high scores by excluding large numbers of patients by exception reporting. More research is needed to determine whether these practices are excluding patients for sound clinical reasons or in order to increase income.

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