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Impact of patient and public involvement on enrolment and retention in clinical trials: systematic review and meta-analysis

BMJ 2018; 363 doi: (Published 28 November 2018) Cite this as: BMJ 2018;363:k4738

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  1. Joanna C Crocker, research fellow1 2,
  2. Ignacio Ricci-Cabello, Miguel Servet research fellow3 4 5,
  3. Adwoa Parker, research fellow6,
  4. Jennifer A Hirst, senior researcher7,
  5. Alan Chant, patient partner2,
  6. Sophie Petit-Zeman, former director of patient involvement2,
  7. David Evans, professor in health services research8,
  8. Sian Rees, patient and public involvement, engagement and experience theme lead9
  1. 1Health Experiences Research Group, Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford OX2 6GG, UK
  2. 2National Institute for Health Research (NIHR) Oxford Biomedical Research Centre (BRC), John Radcliffe Hospital, Oxford, UK
  3. 3Balearic Islands Health Research Institute (IdISBa), Palma de Mallorca, Spain
  4. 4Primary Care Research Unit of Mallorca, Balearic Islands Health Service, Palma de Mallorca, Spain
  5. 5Ciber de Epidemiologia y Salud Publica (CIBERESP), Madrid, Spain
  6. 6York Trials Unit, Department of Health Sciences, University of York, York, UK
  7. 7Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford, UK
  8. 8University of the West of England, Bristol, UK
  9. 9Oxford Academic Health Science Network, Oxford, UK
  1. Correspondence to: J Crocker joanna.crocker{at}
  • Accepted 30 October 2018


Objective To investigate the impact of patient and public involvement (PPI) on rates of enrolment and retention in clinical trials and explore how this varies with the context and nature of PPI.

Design Systematic review and meta-analysis.

Data sources Ten electronic databases, including Medline, INVOLVE Evidence Library, and clinical trial registries.

Eligibility criteria Experimental and observational studies quantitatively evaluating the impact of a PPI intervention, compared with no intervention or non-PPI intervention(s), on participant enrolment and/or retention rates in a clinical trial or trials. PPI interventions could include additional non-PPI components inseparable from the PPI (for example, other stakeholder involvement).

Data extraction and analysis Two independent reviewers extracted data on enrolment and retention rates, as well as on the context and characteristics of PPI intervention, and assessed risk of bias. Random effects meta-analyses were used to determine the average effect of PPI interventions on enrolment and retention in clinical trials: main analysis including randomised studies only, secondary analysis adding non-randomised studies, and several exploratory subgroup and sensitivity analyses.

Results 26 studies were included in the review; 19 were eligible for enrolment meta-analysis and five for retention meta-analysis. Various PPI interventions were identified with different degrees of involvement, different numbers and types of people involved, and input at different stages of the trial process. On average, PPI interventions modestly but significantly increased the odds of participant enrolment in the main analysis (odds ratio 1.16, 95% confidence interval and prediction interval 1.01 to 1.34). Non-PPI components of interventions may have contributed to this effect. In exploratory subgroup analyses, the involvement of people with lived experience of the condition under study was significantly associated with improved enrolment (odds ratio 3.14 v 1.07; P=0.02). The findings for retention were inconclusive owing to the paucity of eligible studies (odds ratio 1.16, 95% confidence interval 0.33 to 4.14), for main analysis).

Conclusions These findings add weight to the case for PPI in clinical trials by indicating that it is likely to improve enrolment of participants, especially if it includes people with lived experience of the health condition under study. Further research is needed to assess which types of PPI work best in particular contexts, the cost effectiveness of PPI, the impact of PPI at earlier stages of trial design, and the impact of PPI interventions specifically targeting retention.

Systematic review registration PROSPERO CRD42016043808.


  • Contributors: JCC, AC, SPZ, DE, and SR conceived and designed this review. JCC, IRC, and AP undertook searches, record screening, and data extraction (supervised by JCC). JH wrote the code for and ran the meta-analyses in Stata. All authors contributed to interpretation of the results. JCC wrote the manuscript, and all authors commented on the draft and approved the final version. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted. JCC is the guarantor.

  • Funding: JC, SPZ, and JH were supported by the National Institute for Health Research (NIHR) Oxford Biomedical Research Centre (BRC). IRC was supported by the University of Oxford Returning Carers Fund. The funders were not involved in the study design; data collection, analysis, or interpretation; or writing the report. All authors had full access to all of the data in the study and can take responsibility for the integrity of the data and accuracy of the data analysis. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR, or the Department of Health and Social Care.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at (available on request from the corresponding author) and declare: no support from any organisation for the submitted work other than that described above; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: Not needed.

  • Data sharing: The dataset is available on request from the corresponding author.

  • Transparency declaration: The lead author (study guarantor) affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.

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