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, it can be concluded that 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 for participant recruitment can be used when developing a research project. Questions that researchers can ask themselves are:
  1. 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 participation articles which are specific for your research design, topic and type of participants. It is preferred to conduct this literature search while writing the research protocol.
    • 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 EMGO 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?
  2. 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.
  3. Project evaluation:
    • Was the recruitment of this project successful yes or no 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?


Report 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 en 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 here.
Research into recruitment and especially determinants of unsuccessful recruitment is common. In 2008, ZonMW published a research performed over the period 2001-2005 among 113 research projects.1 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.




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 indicated below. However, 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.
  • Van Gastel W, red. Het halen en falen van een succesvolle inclusie. Een studie naar inclusievertraging, oorzaken en risicofactoren voor vertraging bij het projecten van het ZonMw programma DoelmarigheidsOnderzoek aangevuld met een quick scan bij het ZonMw programma Preventie. ZonMw rapport 100/11/2008/5.
  • Van der Wouden JC, Blankenstein AH, Huibers MJH, Van der Windt DAWM, Stalman WAB, Verhagen AP. Survey among 78 studies showed that Lasagna’s law holds in Dutch primary care research. Journal of Clinical Epidemiology, 2007;60(8):819-824.
  • Robroek SJW, van Lenthe FJ, van Empelen P, Burdorf A. Determinants of participation in worksite health promotion programmes: a systematic review. International Journal of Behavioral Nutrtition and Physical Activity, 2009;6(26).
  • Lakerveld J, IJzelenberg W, van Tulder MW, Hellemans IM, Rauwerda JA, van Rossum AC, Seidell JC. Motives for (not) participating in a lifestyle intervention trial. BMC Med Res Methodol, 2008; 8(17).


V2.0: 7 July 2015: Revision format
V1.0: 25 Jan 2010