The effects of an open design on trial participant recruitment, compliance and retention--a randomized controlled trial comparison with a blinded, placebo-controlled design

Clin Trials. 2004;1(6):490-8. doi: 10.1191/1740774504cn053oa.


Background: In randomized trials there may be no overriding reason whether or not to have a placebo control.

Purpose: We assessed the effects of an open trial design (no placebo and people know what tablets they are given) compared with a blinded, placebo-controlled design on recruitment, compliance and retention within a randomized trial of secondary osteoporotic fracture prevention.

Methods: We undertook a randomized controlled comparison nested within a placebo-controlled trial of nutritional supplementation amongst people aged 70 years or over who had previously sustained a fracture, recruited in a UK teaching hospital. Randomization was 2:1 in favour of the blinded, placebo-controlled trial design.

Results: From 180 eligible participants randomized to receive information based on the open trial design, 134 (74.4%) consented to take part, compared with 233 (65.1%) of 358 people randomized to the blinded, placebo-controlled design (difference 9.4%, 95% confidence interval 1.3-17.4%). Reluctance to take a placebo and the desire to know tablet allocation were reasons given for not taking part in the blinded, placebo-controlled design. There was no significant difference in tablet compliance. Open trial participants were more likely to remain in the trial for one year (difference 13.9%, 95% confidence interval 3.1-24.6%), mainly reflecting the high retention of the open trial no tablet group compared to the open trial tablet group (difference 23.6%, 95% confidence interval 11.9-35.2%). The odds ratio for reporting an adverse event in the open trial compared to the blinded, placebo-controlled design was 0.64 (95% confidence interval 0.28-1.49), and for reporting a fracture was 0.81 (0.36-1.85).

Conclusions: We conclude that using an open trial design may enhance participant recruitment and retention and thus improve generalizability and statistical power, but withdrawal rates may differ between the study allocations and may threaten the internal validity of the trial.

Publication types

  • Clinical Trial
  • Comparative Study
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aged
  • Bone Density Conservation Agents / therapeutic use
  • Calcium / therapeutic use
  • Dietary Supplements
  • Double-Blind Method
  • Female
  • Fractures, Spontaneous / etiology
  • Fractures, Spontaneous / prevention & control*
  • Humans
  • Male
  • Osteoporosis / complications*
  • Patient Compliance*
  • Patient Dropouts
  • Patient Selection*
  • Refusal to Participate
  • Research Design*
  • Vitamin D / therapeutic use


  • Bone Density Conservation Agents
  • Vitamin D
  • Calcium