Sharp 2006.

Study characteristics
Methods	2 x 2 x 2 factorial parallel RCT, Individuals randomised
Data	UK, secondary care setting. Participants due to receive a host trial questionnaire were involved. Total n = 930; mean age 34 (SD 10.4); 100% females; 96% White.
Comparisons	Three trials were evaluated in this study: Trial 1: Intervention group received a TOMBOLA‐branded pen Control group received no pen Trial 2: Intervention group questionnaire was dispatched by first class post Control group questionnaire was dispatched by second class Trial 3: Intervention group received an enclosed pre‐addressed return envelope on which there was a second‐class postage stamp Control group received Freepost business‐reply envelope. This generated eight intervention groups: 1. standard (i.e. no pen, second class dispatch, Freepost return envelope) 2. pen 3. pen and first‐class dispatch 4. first‐class dispatch 5. stamp on the return envelope 6. stamp on the return envelope and pen 7. stamp on the return envelope and first‐class dispatch stamp on the return envelope, pen and first‐class dispatch
Outcomes	Questionnaire response rate
Notes	Retention period: 12‐, 18‐, 24‐ and 30‐months questionnaires
Risk of bias
Item	Authors' judgement	Support for judgement
Allocation concealment?	Unclear	Not reported in the paper.
Adequate sequence generation?	Yes	Computer randomised by two authors using random numbers, to one of the eight intervention groups
Blinding of participants and personnel?	Yes	Not reported in the paper. Not clear if participants were blinded to effect of embedded trial intervention on retention and unblinding but likely to impact objective outcome.
Blinding of outcome assessment?	Yes	Staff not blind but they had no influence on a participant's decision to reply
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