Cunningham‐Burley 2020.

Study characteristics
Methods	Parallel RCT, individuals randomised
Data	UK, secondary care setting. Participants in the host trial who were due to be sent their 14‐week postal questionnaire. Total n = 1466, mean age 43.0 (SD 11.3) years, sex 86.1%.
Comparisons	Intervention group received a branded pen with their questionnaire. Control group did not receive a pen.
Outcomes	Proportion of participants who return the questionnaire.
Notes	Retention period: 14‐week questionnaire
Risk of bias
Item	Authors' judgement	Support for judgement
Allocation concealment?	Unclear	Not discussed in the paper
Adequate sequence generation?	Yes	Participants were allocated to either the intervention (pen) or control (no pen) group using simple randomisation in a 1:1 ratio. The allocation sequence was generated by the host trial statistician, who was not involved in sending out the questionnaires.
Blinding of participants and personnel?	Yes	Participants were not aware of their involvement in this SWAT, but due to the nature of the intervention participants and study team members could not be blinded to group allocation.Unblinding not likely to impact objective outcome
Blinding of outcome assessment?	Yes	Not discussed in the paper.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