Marson 2007.

Study characteristics
Methods	Parallel RCT, individuals randomised
Data	UK, secondary care setting. It is unclear if all participants from host trial were involved. Total n = 1815; age NR; sex NR
Comparisons	Intervention group received a cover letter though post with the questionnaire that included an estimate of the length of time that it may take to complete. Control group received standard cover letter with no indication of length of time required.
Outcomes	Return of questionnaire.
Notes	Retention period: baseline assessment
Risk of bias
Item	Authors' judgement	Support for judgement
Allocation concealment?	Unclear	Not reported in the monograph.
Adequate sequence generation?	Unclear	Not reported in the monograph.
Blinding of participants and personnel?	Yes	Not reported in the monograph but unblinding not likely to impact objective outcome
Blinding of outcome assessment?	Yes	Not reported in the monograph.
Incomplete outcome data addressed? All outcomes	Unclear	Not reported in the monograph. However, objective outcome, staff have no plausible additional opportunity to influence postal response rate once questionnaires sent.
Free of selective outcome reporting?	Unclear	Not reported in the monograph.
Other sources of bias	Yes	No further concerns raised.
Overall Risk of Bias	Unclear	Unclear