Table 4. Secondary outcome results.

Secondary outcome	Group	Hazard ratio (HR)/ Odds ratio (OR)/Mean difference (MD)	95% Confidence Interval	p-value	Other
Time to return	Pen received vs. not received	HR = 1.08	0.93, 1.25	0.30	Mean time for all participants to return questionnaire = 12.2 days. Median time for all participants to return questionnaire = 9 days.
	Social incentive cover letter received vs. not received	HR =1.101	0.87, 1.17	0.92
	Host trial allocation (intervention vs. control)	HR = 0.85	0.73, 1.00	0.05
	Age (per year)	HR = 0.99	0.97, 1.00	0.02
	Gender (male vs. female)	HR = 1.80	0.92, 1.26	0.35
Reminders sent	Pen received vs. not received	OR = 0.89	0.56, 1.42	0.63	83/755 (11.0%) required a reminder p value associated with the Kruskal- Wallis test statistic p=0.190
	Social incentive cover letter received vs. not received	OR = 0.92	0.58, 1.47	0.74
	Host trial allocation (intervention vs. control)	OR = 1.611	1.00, 2.59	0.05
	Age (per year)	OR = 1.04	1.00, 1.08	0.03
	Gender (male vs. female)	OR = 0.87	0.53, 1.42	0.57
Completeness of response	Pen received vs. not received	MD = 0.14	-0.46, 0.74	0.65	Overall average completeness of the questionnaires was 27.8/31 questions (89.6% complete)
	Social incentive cover letter received vs. not received	MD = 0.09	-0.69, 0.51	0.78
	Host trial allocation (intervention vs. control)	MD = -0.10	-0.55, 0.75	0.77
	Age (per year)	MD = -0.10	-0.46, 0.74	0.65
	Gender (male vs. female)	MD = -1.06	-1.69, -0.42	˂0.001