Study population

The study comprised all participants who contributed acceptable quality data to the CPRD between 1 January 1990 and 31 December 2014. The denominator for prevalence estimation for each calendar year included all individuals registered on 1 July of each calendar year with a general practitioner (GP). For the incidence of RA, at-risk cohorts for each calendar year were constructed, which comprised all individuals registered with up-to-standard practices (practices meeting the specified data entry criteria) during the year specified who had never been coded as having RA before the latest of current registration date plus 365 days or 1 January of the calendar year specified. Person-years of follow-up were calculated from the latest of 1 January or the date of current registration plus 365 days to the earliest date of transfer out, incident RA diagnosis, death or 31 December of the specified year.