Evidence Summary

What is it?

Mentioning scarcity of trial places in text messages.

Does it work?

Mentioning scarcity of trial places in text messages probably increases recruitment slightly.

How big is the effect?

An increase of 3% (95% confidence interval = 1% to 6%).

How certain are we?

GRADE Moderate certainty.


We suggest that trialists consider using text messaging #1: mentioning scarcity, ideally as part of a SWAT evaluation

How can I use this straight away?

See Resource bundle below for details of how to use text messaging #1: mentioning scarcity or build a SWAT evaluation of them into your trial.

Practical Impact

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).


Now imagine mentioning scarcity in a text message. The chart below shows the impact of an absolute increase of 3% (95% CI = 1% to 6%). Recruitment is now 3%, which means our best estimate is that 91 people would now need to be approached to recruit 30 of them.

Cumulative Meta-Analysis*

*Random effects model done using Comprehensive Meta-Analysis v3 (www.meta-analysis.com).
Differences >0% favour the intervention. The GRADE assessment is moderate because of imprecision due to being a single study.

Resource Bundle


How to Cite

Citation: Treweek S, Bruhn H, Gardner H. Evidence pack Recruitment: Text messaging #1: mentioning scarcity (Rec 6), 2021, https://www.trialforge.org/recruitment-sector/mentioning-scarcity-trial-places-sms-messages/.


More Information

  1. This summary is from the Cochrane review of strategies to improve recruitment in randomised trials (https://doi.org/10.1002/14651858.MR000013.pub6).
  2. It was prepared with financial support from Evidence Synthesis Ireland.
  3. The ‘Does it work?’ statement is structured according to effect size and GRADE certainty as per GRADE Guidelines 26 (https://doi.org/10.1016/j.jclinepi.2019.10.014). The statement is for a small important effect and GRADE moderate certainty.
  4. The recommendation statement is the consensus view of the authors of this summary based on the GRADE certainty and features of the trials contributing to the evidence.
  5. If you have any questions contact info@trialforge.org.
Version: v1.0 - 22/12/2021

Start typing and press Enter to search