Spirometry

The study period extended from 1 April 2004, the date of introduction of the incentivisation scheme, to 31 March 2012. Practices’ performance under the QOF is measured over a financial year, so we divided the study period into eight financial years (1 April to 31 March the following year). Not all 644 practices provided research standard data (as assessed by the CPRD assessment algorithm) for the whole period. Within each year, we identified practices that reliably contributed data for the whole year. Our main dataset comprised this group of practices, which varies over time. We also generated two alternative datasets with which to assess the sensitivity of our findings. For the first, we included 452 practices that were continuously active and up to standard for the whole of the study period; for the second, we selected a subsample of 50 practices that were most representative of UK practices in terms of list sizes of patients and area deprivation according to the Index of Multiple Deprivation,13 14 two of the most important predictors of QOF performance.12 15 16 In each of the three datasets, for each financial year, we defined “eligible” patients as those registered with an included practice for the full year.