Mitchell 2020a.

Study characteristics
Methods	Parallel RCT, Individuals randomised
Data	UK, secondary care setting. Participants from the host trial who provided they had opted in to receiving SMS messages and were not deceased or withdrawn from follow‐up before being due to be sent their 12‐month postal questionnaire Total n = 1465; mean age 66.8 (8.5); 54.0% females.
Comparisons	Intervention group received a personalised text message four days after their 12‐month questionnaire was sent. Control group received a non‐personalised text message.
Outcomes	Questionnaire return
Notes	Retention period: 12 months
Risk of bias
Item	Authors' judgement	Support for judgement
Allocation concealment?	Yes	Participants were randomised into the embeded trial using simple randomisation in a 1:1 allocation ratio. The allocation schedule was generated by a researcher at the Trials Unit not involved in the recruitment or follow‐up of participants.
Adequate sequence generation?	Yes	Participants were randomised into the embeded trial using simple randomisation in a 1:1 allocation ratio. The allocation schedule was generated by a researcher at the Trials Unit not involved in the recruitment or follow‐up of participants.
Blinding of participants and personnel?	Yes	Participants were not informed of their explicit participation in the embeded trial, but due to the nature of the intervention could not be blinded to whether the text was personalised or non‐personalised. Unblinding not likely to impact objective outcome.
Blinding of outcome assessment?	Yes	It was not possible to blind research staff to SWAT allocation. However, objective outcome, participants blinded (did not know there was a study), 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	Yes	Low