Table 1.

RCTs of opt-in and opt-out consent

Study population and purpose	Parents in a health district of the UK were asked for consent for inclusion of low birth-weight infants on a register for the purpose of monitoring disability in children [26]	Mothers in the US were asked for consent for inclusion of infants at high risk to participate in a clinical trial of primary follow-up care [27]	Angina patients in two general practices in the UK were asked for consent to be involved in clinical research [28]	Patients aged 50-74 years in a general practice in Australia were asked to consent to testing decision aids for the screening of colorectal cancer [29]	Cancer patients in the Netherlands who had undergone primary surgery were asked for consent for the storage of excised tissue for future research purposes [30]
Sample size randomised (n)	Opt-in: 39 Opt-out: 30	Opt-in: 32 Opt-out: 25 (3 were excluded as they did not receive the allocated intervention)	Opt-in: 252 Opt-out: 258	Opt-in: 92 Opt-out: 60	Opt-in: 60 'Opt-out plus': 73 Control group (standard opt-out): 131
Mode of invitation	Verbal information, letter and reply slip given by a nurse prior to an infant's discharge from hospital	Verbal information and reply form given by a nurse within 24-48 hours of delivery. The opt-out form was shortened to include only specific disclosures that are appropriate for low risk research	Letter, information leaflet and reply card sent from a doctor	Letter sent from a doctor (plus reply card for the opt-in arm only)	Verbal information, specific information leaflet and reply form given by a doctor/nurse. The control group was only given a routine hospital leaflet and did not receive verbal information
Mode of response	Reply-paid slip	Reply form was collected from the mother	Reply card or telephone	Telephone or email (or reply-paid card for the opt-in arm)	Reply-paid form. The control group leaflet instructed patients to opt out by informing their doctor
Reminder letter	No	No	After two weeks for the opt-in arm only	No	No
Time to respond	Not stated	Prior to discharge from hospital. Once a mother reached a decision, an interview occurred within the next 24 hours (usually 2 hours)	Opt-in: Not stated Opt-out: patients could opt out verbally when telephoned after two weeks	Not stated	One month
Participation rate	Opt-in: 79% Opt-out: 97%	Opt-in: 75% Opt-out: 91%	Opt-in: 48% Opt-out: 59%	Opt-in: 51% Opt-out: 90%	Opt-in: 85% 'Opt-out plus': 97% Standard opt-out: 100%
Recruitment rate	Not applicable	Face-to-face interview Opt-in: 81% Opt-out: 82%	Clinic attendance Opt-in: 38% Opt-out: 50%	Telephone survey Opt-in: 47% Opt-out: 67%	Postal and telephone survey Opt-in: 93% and 52% 'Opt-out plus': 93% and 51% Standard opt-out: 88% and 47%
Evidence of selection bias	Not stated	Modest differences were found. Subjects recruited in the opt-in arm were older, more likely to be married and undergo a vaginal delivery than subjects in the opt-out arm	Subjects recruited in the opt-in arm were healthier and had less risk factors for coronary disease than subjects in the opt-out arm	Subjects recruited in the opt-in arm were more likely to prefer an active role in decision making than subjects in the opt-out arm	Subjects recruited in the opt-in arm were similar in age, sex, education and type of cancer to the 'opt-out plus' arm. The control group was similar, except that women were over-represented
Design flaws	Small sample size, non-random allocation and no mention of whether blinding was used	Small sample size and the collection of reply forms is resource-intensive and impracticable on a large scale	None evident	Small sample size and non-parallel design	Small sample size and no mention of whether blinding was used