Opt-out rather than opt-in consent

Using opt-out consent rather than opt-in consent may improve recruitment.


The absolute increase is 19% (95% CI = 3% to 35%)1

How confident are we in the evidence?
What is GRADE?

LOW CERTAINTY in the evidence (but see 'What we don't know' below)

1 Treweek S, Pitkethly M, Cook J, Fraser C, Mitchell E, Sullivan F, Jackson C, Taskila TK, Gardner H. Strategies to improve recruitment to randomised trials. Cochrane Database of Systematic Reviews 2018, Issue 2.

The practical impact of opt-out consent

Imagine a trial that needs to recruit 30 participants and initial recruitment is 30% of those approached. This means you'd need to approach 100 people to recruit 30 of them (see chart below).

Opt-in consent
With opt-in consent

Now imagine using a opt-out consent. The chart below shows the impact of an absolute increase of 19% (95% CI = 3% to 35%)1. Recruitment is now 49%, which means our best estimate is that 61 people would need to be approached to recruit 30 of them.

With opt-out consent
With opt-out consent
By using opt-out consent, our best estimate is that you will need to approach 39 fewer participants.

Where has opt-out consent been tested?


Opt-out consent


Opt-in consent


1 study including a total of 152 participants

Study 1: Trevena 2006

Participants were aged 50 to 74 and eligible for a colorectal cancer screening trial.

Trial intervention?
Decision aid.

Study location?
Primary care, Australia.

What difference?
66.6% of those allocated to opt-out consent were recruited; 46.7% of those allocated to opt-in consent were recruited.

Read more about this study here.

What we still don’t know about op-out consent

  • The information we have is based on a single trial. The distribution of participants between intervention and comparison groups is uneven: 60 versus 92, respectively. This was due to a change in legislation in Australia, which meant that the trialists could no longer continue with the opt-out procedure and had to change to opt-in to keep their ethical approval. Ideally we need to test this intervention in more trials.
  • Please get in touch (email info@trialforge.org) if you would like to do an evaluation because we can help with text for ethics etc.

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