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.

Against a background of increases in the quality of care before the pay-for-performance scheme was introduced, the scheme accelerated improvements in quality for two of three chronic conditions in the short term. However, once targets were reached, the improvement in the quality of care for patients with these conditions slowed, and the quality of care declined for two conditions that had not been linked to incentives. Continuity of care was reduced after the introduction of the scheme.

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