Table 1.

Costs associated with recruitment, participation, and retention by initial recruitment method.

Phase	Information	Total cost (US $)	Cost description	Unique participants paid, n	Participants retained, n (%)a	Nominal cost per participant (US $)	Realized (adjusted) cost per participant (US $)	Cost difference (US $)
Advertising recruitment	Facebook and Instagram	26,571.62	18,469 clicks	2143	2022 (10.9b)	1.44	13.14	11.69
Advertising recruitment	YouTube	1892.89	1621 clicks	81	78 (4.8c)	1.16	24.27	23.11
RDSd recruitment	Referrals	4730	473 referrers at US $10 per referral	508	458 (90.2e)	10.00	10.33	0.33
Study enrollment	Baseline	40,980	US $15 baseline survey incentive	2732	2558 (93.6)	15.00	16.02	1.02
Retention	6 months	19,940	US $20 6-month survey incentive	997	969 (97.2)	20.00	20.58	0.56
Retention	12 months	24,025	US $25 12-month survey incentive	961	945 (98.3)	25.00	25.42	0.40
Retention	18 months	27,570	US $30 18-month survey incentive	919	912 (99.2)	30.00	30.23	0.20
Retention	24 months	31,430	US $35 24-month survey incentive	898	894 (99.6)	35.00	35.16	0.12
Retention	Raffles	3100	US $100 monthly raffle prize	31	31 (100)	100.00	100.00	0.00

^aThe denominator of each percentage is the number of unique participants paid in the same row, except for those that indicate otherwise.

^b10.9% of attempts out of 18,469 clicks.

^c4.8% of attempts out of 1621 clicks.

^dRDS: respondent-driven sampling.

^e90.2% of referrals out of 473 referrers at US $10 per referral.