23 Patients from linked practices are also representative of the entire CPRD population.
The subset of linked General Practice Research Database practices is broadly similar in profile to the full complement of practices, suggesting generalisability is maintained when the linked practices alone are used.
However, the patients registered in the linked practices have been shown to be representative of the whole CPRD population [ 53].
We limited the main analysis to patients who initiated treatment with a smoking cessation medicine from linked practices within the study period.
There were 81 545 patients prescribed nicotine replacement therapy (68.2%), 6741 for bupropion (5.6%) and 31 260 for varenicline (26.2%) in the linked practices (table 1).
High active participants cited the importance of 'exercise', which supports previous research [ 61] that found that students' responses similarly linked practices like eating and exercise with being healthy.
We identified 1063 suicides using Read code algorithms in these linked practices; 590 of these [55.5%, 95% confidence interval (CI): 52.5 58.5%] were true, ONS-confirmed suicides (PPV).
Any linked practices (those practices that have the same GPs across two sites, each with a different patient list) will be stratified so that the allocation to trial arm will be the same in order to avoid contamination.
The most effective defense is to link practice of skills in the workforce as closely as possible to the context in which they were learned.
Finally, we link practice codes to the Health and Social Care Information centre database containing numerators, denominators and number of points that each practice has obtained on all QOF indicators from 2004 onwards.
