Randomisation
A parallel-arm design with 1:1 allocation was used, with 18,247 doctors randomly allocated to either the control or intervention group. Allocation was stratified by doctor type (GP, specialist), continuing or new, and boost sample to ensure the proportions of these groups of doctors in the intervention and control groups were the same. This is important because new doctors and boost sample doctors are likely to have lower response rates, and specialists had higher response rates than GPs in previous waves. We tested a two-sided hypothesis as it was unclear, a priori, whether the intervention group would have a higher or lower response rate. Randomisation was performed using the Randomisation took place (by TT) before the first invitation for Wave 11 was mailed out in August 2018. Group allocation was kept separate (in a separate electronic data file) from the main mailout and the first reminder so researchers handling the responses and reminders were blinded to group allocation during this process. The second reminder was prepared in late November 2018 by TT. The list of those eligible for a second reminder was then merged with the file containing the intervention and control group identifiers to indicate who should receive an email. A separate file indicating whether doctors had an email address was also merged onto this file. AMPCo was sent a list of doctor identifiers indicating if they should be approached using a mailed letter or an email.