The effect of using social pressure in cover letters to improve retention in a longitudinal health study: an embedded randomised controlled retention trial

Table 2 Results: questionnaire return by experimental condition, using logistic regression adjusting for household and baseline characteristics and clustering

Outcome		Intervention N = 2223 n (%)	Control N = 2224 n (%)	Risk difference (unadjusted) (%)	Odds ratio (95% CI) Intervention vs. control	p value
Primary	Both questionnaires returned	1577 (71)	1511 (68)	3	1.15 (1.01–1.31)	0.030
					1.15^a (1.01–1.31)	0.036
					1.16^b (1.02–1.33)	0.021
Secondary	At least one of questionnaires returned	1809 (81)	1787 (80)	1	1.07 (0.92–1.24)	0.385
					1.07^a (0.90–1.28)	0.463
					1.07^b (0.92–1.25)	0.348
	Six-month questionnaire returned	1739 (78)	1701 (76)	2	1.10 (0.95–1.27)	0.165
					1.10^a (0.94–1.30)	0.233
					1.11^b (0.96–1.28)	0.149
	12-month questionnaire returned	1647 (74)	1597 (72)	2	1.12 (0.98–1.28)	0.087
					1.12^a (0.98–1.28)	0.086
					1.14^b (0.99–1.29)	0.062

ISSN: 1745-6215