OPAL 2020.

Study characteristics
Methods	Parallel RCT, individuals randomised.
Data	UK, secondary care settings. not ALL participants in the trial that took part in the trial. Only those due to be receiving their 12‐ or 24‐month questionnaire at the time the trial was running.
Comparisons	Intervention group received a tested theoretically informed letter sent with the questionnaire Control group received a standard letter
Outcomes	Questionnaires returned
Notes	12 or 24 months
Risk of bias
Item	Authors' judgement	Support for judgement
Allocation concealment?	Unclear	No information of the nested RCT was provided.
Adequate sequence generation?	Yes	The database would automatically produce the appropriate letter for each woman
Blinding of participants and personnel?	Yes	Women were unaware if they were receiving a standard or theory‐based cover letter but unblinding not likely to impact objective outcome
Blinding of outcome assessment?	Yes	No information of the nested RCT was provided.However, objective outcome, staff have no plausible additional opportunity to influence postal response rate once questionnaires sent.
Incomplete outcome data addressed? All outcomes	Yes	No concerns raised.
Free of selective outcome reporting?	Yes	No concerns raised.
Other sources of bias	Yes	No further concerns raised.
Overall Risk of Bias	Unclear	Unclear