Table 1.

Summary of evidence from the Cochrane systematic review of strategies to improve retention in RCTs and qualitative study on the use of retention strategies in RCTs

	Systematic review results
	Method of data collection	Number of RCTs in meta-analysis	Total number of participants in meta-analysis	RR 95% CI	P value	Absolute benefit based on 50% baseline response	Qualitative study results
Effective retention strategies
Monetary incentives
Addition of monetary incentive vs. none [15–17]	Postal questionnaire	3	3,166	RR 1.18; 1.09–1.28	P < 0.0001	76 questionnaires per 1,000 sent	Incentives are used in cash or voucher format given up front or on questionnaire completion. General agreement that small monetary incentives are viewed favorably by ethics committees. Uncertainty about effect of monetary incentives given up front or offered for questionnaire return.
Offer of a monetary incentive vs. none [18]a	Web-based questionnaire	2	3,613	RR 1.25; 1.14–1.38, heterogeneity P value = 0.14	P < 0.00001	100 questionnaires per 1,000 sent	Offers of monetary incentives used.
Higher value monetary incentive vs. lower value monetary incentive (Bailey unpublished)	Postal questionnaire	2	902	RR 1.12; 1.04–1.22	P = 0.005	55 questionnaires per 1,000 sent	£5–£20 monetary incentives used. Concern about coercion with higher valued incentives.
Strategies with some evidence of effect based on single RCTs
Communication
Total Design Method (TDM) vs. customary postal communication [8]	Postal questionnaire	1	226	RR 1.43; 1.22–1.67	P < 0.0001	—	Some elements of TDM used to improve postal questionnaire response.
Recorded delivery vs. telephone reminder [9]	Postal questionnaire	1	192	RR 2.08; 1.11–3.87	P = 0.02	—	Recorded delivery used to send further copy of questionnaire/study materials. Mixed opinions on usefulness.
Methodology strategies Open vs. blind RCT design [19]	Postal questionnaire	1	538	RR 1.37; 1.16–1.63	P = 0.0003	—	Open trial design not used to improve retention. Masking RCT participants to the intervention used to avoid bias associated with open RCTs.
Strategies with unclear evidence of effect
New questionnaire strategies
Short questionnaires vs. long (Edwards unpublished, Svoboda, unpublished) [20,21]a	Postal questionnaire	5	7,277	RR 1.04; 1.00–1.08	P = 0.07	20 questionnaires per 1,000 sent	Shorter follow-up questionnaires used with a second reminder. Long questionnaires thought to be off putting for participants.
More relevant questionnaires (i.e., those relating to alcohol use) vs. less relevant [21]a	Web based	2	3,893	RR 1.07; 1.01–1.14	P = 0.03	—	No comments on the use of more or less relevant questionnaires.
Noneffective strategies
Nonmonetary incentives
Addition of nonmonetary incentive vs. none [22–24]	Postal questionnaire	6	6,322	RR 1.00; 0.98–1.02, some heterogeneity (P value = 0.02)	P = 0.91	—	Gifts used as reminders about RCTs. Uncertainty about effectiveness.
Offer of a nonmonetary incentive vs. no offer [25,26]	Postal questionnaire	2	1,138	RR 0.99; 0.95–1.03	P = 0.60	—	Offers of gifts not mentioned as a strategy to improve retention.
Addition of monetary incentive vs. offer of prize draw entry [18]	Postal questionnaire	2	297	RR 1.04; 0.91–1.19	P = 0.56	—	Offers of entry into a prize draw seldom used but thought to potentially be useful.
Offer of monetary donation to charity vs. none [18]	Web-based questionnaire	1	815	RR 1.02; 0.78–1.32	P = 0.90	—	Offers of donations to charity not mentioned as a way to improve retention.
Communication strategies
Enhanced letter vs. standard letter [23,27]	Postal questionnaire	2	2,479	RR 1.01; 0.97–1.05	P = 0.70	—	Enhanced letter routinely used to improve questionnaire return.
Priority post vs. regular post [23,24,28]	Postal questionnaire	7	1,888	RR 1.02; 0.95–1.09	P = 0.55	—	First-class post routinely used to send post to participants.
Additional reminder vs. usual follow-up practices [29–32]a	Postal questionnaire	6	3,401	RR 1.03; 0.99–1.06	P = 0.13	—	SMS text reminders thought useful for contacting young RCT participants. Thought similar system used for text reminders for NHS clinic appointments may improve follow-up in RCTs. Telephone reminders routinely used. Concerns about harassment with too many reminders. E-mail reminders thought useful for improving response.
Early vs. late questionnaire administration [23]	Postal questionnaire	1	664	RR 1.10; 0.96–1.26	P = 0.19	—	Questionnaires sometimes posted later in week to arrive at weekend.
Additional monthly reminder to RCT site vs. usual reminder (Land unpublished)	Return to research site	1	272	RR 0.96; 0.83–1.11	P = 0.57	—	Additional reminders to sites not mentioned as a way to improve retention.
Addition of telephone survey vs. monetary incentive plus questionnaire [33]	Postal questionnaire	1	700	RR 1.08; 0.94–1.24	P = 0.27	—	Telephone survey seldom used to improve retention. Telephone calls used by nurses to contact participants.
New questionnaire strategies
Disease/condition questions before generic vs. generic questions before disease/condition questions [34]a	Postal questionnaire	2 quasi-randomized	9,435	RR 1.00; 0.97–1.02	P = 0.75	—	Suggestions to improve questionnaire format include: < 10 pages, clear succinct questions, avoid repetition, include participant feedback section, use illustrations, color coordinate questionnaires for each time point.
Long and clear questionnaires vs. shorter condensed questionnaires [35]	Postal questionnaire	1	900	RR 1.01; 0.95–1.07	P = 0.86	—	Shorter questionnaires used where possible.
Behavioral/motivational strategies
Behavioral/motivational strategies vs. standard information [36,37]	Return to research site	2	273	RR 1.08; 0.93–1.24	P = 0.31	—	Not used, very negative about the usefulness of using behavioral strategies for retention.
Case management
Case management vs. usual follow-up [38]	Return to research site	1	703	RR 1.00; 0.97–1.04	P = 0.99	—	Case management, seldom used, thought to be potentially useful for retention but expensive.

Abbreviations: RCT, randomized clinical trial; CI, confidence interval; RR, risk ratio.

^aPublication reports more than one retention RCT.