Successful recruitment of study participants

Inadequate recruitment is the most common reason for premature termination of randomised clinical trials (RCTs). This is particularly true for academic trials initiated by sponsor-investigators [1].

The reasons for recruitment failure in RCTs are multiple and often interrelated [2,3]. Many causes are considered preventable and therefore should be appropriately considered when planning clinical trials [4].

Below table summarises the 10 most common causes of inadequate recruitment [3] and suggests measures to address them in order to reduce the risk of missing recruitment goals. Here you can download the table as a pdf.

Reasons for insufficient recruitment and what can be done about it:


1. Number of recruitable individuals is overestimated

  • Perform estimation of realistic recruitment numbers based on data from routine clinical practice
  • Run through the recruitment process in practice
2. Inclusion/exclusion criteria are defined too narrowly
  • Carefully consider of all inclusion and exclusion criteria
    • Are all criteria really necessary?
    • What impact do the individual criteria have on the number of possible participants?
    • Define as few exclusion criteria as possible
  • Info sheet "Definition of inclusion/exclusion criteria" (soon available)

3. Recruitment is not coordinated well enough with routine procedures in the clinic
4. Administrative hurdles and time required is too high for the study team or priorities are not clear / Effort for participants is too high

  • Adapt the study procedures to the routine procedures in the clinic
    • Is it clear who is responsible for what, when and where?
    • Can all responsible persons do their work for the study in addition to routine tasks?
    • Do the processes work in all centres?
    • Do the study visits run smoothly from the participants' point of view?
    • Do participants have additional burdens that could perhaps be avoided?
5. Commitment/collaboration in the study team is lacking
6. Identification with the research question is insufficient (equipoise)
  • Avoid "trial fatigue" through professional project management and careful communication
  • Openly discuss identification with the study question with all study centres (decision-makers and recruiters) before the start of the study and question the participation of the centre in case of reservations
7. Initial funding is insufficient
  • Carry out budget planning and control
8. Other studies recruit the same patient population or require the same resources
  • Search for ongoing studies at participating study sites
9. Study design is too complex to explain and to implement
10. Methodological and logistical support is lacking
  • Keep study design as simple as possible
    • Is the focus on the original research question?
    • Are all secondary endpoints really necessary?
    • How complex is the practical implementation of the study?
    • Can the study be integrated into everyday clinical practice?
    • Are there sufficient human resources?