Comparison 24. B‐ Monetary incentives: Addition of monetary reward vs usual follow‐up.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
24.1 Retention	3	1159	Risk Difference (M‐H, Random, 95% CI)	0.02 [‐0.03, 0.06]
24.1.1 Return of postal questionnaires	2	725	Risk Difference (M‐H, Random, 95% CI)	0.00 [‐0.06, 0.07]
24.1.2 Attendance at follow up visits	1	434	Risk Difference (M‐H, Random, 95% CI)	0.03 [‐0.04, 0.10]
24.2 Retention ‐ sensitivity analysis removing quasi‐RCTs	2	955	Risk Difference (M‐H, Random, 95% CI)	0.01 [‐0.04, 0.06]
24.2.1 Return of postal questionnaires	1	521	Risk Difference (M‐H, Random, 95% CI)	‐0.02 [‐0.09, 0.06]
24.2.2 Attendance at follow up visits	1	434	Risk Difference (M‐H, Random, 95% CI)	0.03 [‐0.04, 0.10]