Recruiting participants


To increase awareness among researchers about problems among recruiting participants.

How to

Successfully recruiting research participants is a key aspect for the success of a research project. However, recruitment problems are common within research projects. Due to recruitment problems, inclusion periods are often extended, causing additional (financial) problems. Therefore, researchers are advised to be aware of recruitment problems during different stages of the research project. Moreover, we advise, especially senior researchers, to evaluate every research project carefully and to use the outcomes regarding recruitment problems in future projects. A checklist (see under Download) for participant recruitment can be used when developing a research project.

  1. Questions that researchers can ask themselves are:
  • Designing the study and writing the protocol:
    • What are the risks of this project (based on research design) in terms of recruitment problems? It is advisable to perform a literature search on recruitment specific for your research design, topic and type of participants. It is preferred to conduct this literature search while writing the research protocol.
    • Think about intersectionality, demographics and diversity aspects, like education level, areas, income, age, nationality, cultural diversity, religion, urban vs rural etc.
    • Which methods can I use to try to increase the success of the recruitment?
    • Are the goals related to the number of participants to be included and the recruitment period achievable?
    • Who can I ask for advice about the recruitment of my project? (For example, look at the personal pages on the APH website who has performed research with a similar design).
    • Am I familiar with any resistance among recruiters or participants against the intervention or control conditions?
  1. Data collection:
  • What is the recruitment progress (evaluate regularly)?
  • What are the main causes of recruitment problems (divided into two main categories: a) related to the research/researchers, b) related to the potential research participants).
  • Which methods can I use to increase the participation rate? Think beforehand about alternative or additional recruitment methods.
  1. Project evaluation:
  • Was the recruitment of this project successful or not and what can I learn for future projects?
  • In case of unsuccessful recruitment: what were the consequences in terms of costs and achieved research goals?
Reports on recruitment problems

The Quality Committee of the EMGO+ Institute started an investigation in 2006 to determine to what extent EMGO+ research projects were able to successfully recruit participants and what the possible determinants and outcomes of successful and unsuccessful recruitments were. Of the 61 research projects included, one third included less than 90% of the needed participants and 60% of the projects had to extend the inclusion period. As a result, at least 22% of the projects exceeded their research budget. In addition, researchers reported to have withdrawn part of the research protocol, to have skipped/unanswered research questions and to have shortened the follow-up period. In most projects, methods to increase the inclusion were included in the initial research protocol, of which most used methods were sending reminders, regularly contacting recruiters and handing out small attentions to participants. Researchers reported different potential determinants for unsuccessful recruitment, which were related to the participants as well as to the research project/researcher. For example, recruitment during practice hours and resistance of either the recruiter or the potential participants against the intervention were risk factors.  However, due to the heterogeneity of the research designs and the low sample size, definite conclusions about determinants of (un)successful recruitment could not be drawn from this investigation. The report is (in Dutch) available under Download.

Research into recruitment and especially determinants of unsuccessful recruitment is common. In 2008, ZonMw published a study performed over the period 2001-2005 among 113 research projects (Van Gastel 2008). Of these projects, 49% included less than 80% of the needed participants. Four main risk factors were identified: 1) recruitment in more than one clinical center, 2) RCT as research design, 3) clinical intervention based on treatment, 4) non-clinical intervention based on treatment. Based on these risk factors, ZonMw tried to predict which of the 2006 projects would have an unsuccessful recruitment. This revealed that in projects with more than 2 of these risk factors, the chance of an unsuccessful recruitment was high.Van der Wouden et al (2007a,b) studied recruitment problems in 78 studies in primary care.

See under Download
  • Checklist for recruiting participants (Dutch)
Additional literature

Many articles have been published on the topic of research participation and especially about reasons for not wanting to participate. Some of this literature is listed below.  We advise all researchers to perform a literature search on participation articles which are specific for their research design, topic and type of participants. It is preferred to conduct this literature search while writing the research protocol and use the outcomes to create a future plan for potential recruitment problems.