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Reporting of sample size calculation in randomised controlled trials: review

BMJ 2009; 338 doi: (Published 12 May 2009) Cite this as: BMJ 2009;338:b1732
  1. Pierre Charles, research fellow in epidemiology, specialist registrar in internal medicine123,
  2. Bruno Giraudeau, assistant professor of statistics1456,
  3. Agnes Dechartres, research fellow in epidemiology123,
  4. Gabriel Baron, statistician123,
  5. Philippe Ravaud, professor of epidemiology123
  1. 1INSERM, U738, Paris, France
  2. 2Université Paris 7 Denis Diderot, UFR de Médecine, Paris
  3. 3AP-HP, Hôpital Bichat, Département d’Epidémiologie, Biostatistique et Recherche Clinique, Paris
  4. 4INSERM Centre d’Investigation Clinique 202, Tours, France
  5. 5Université François Rabelais, Tours
  6. 6CHRU de Tours, Tours
  1. Correspondence to: P Ravaud, Département d’Epidémiologie, Biostatistique et Recherche Clinique, Secteur Claude Bernard, Hôpital Bichat Claude Bernard, 75877 Paris, cedex 18, France philippe.ravaud{at}
  • Accepted 2 January 2009


Objectives To assess quality of reporting of sample size calculation, ascertain accuracy of calculations, and determine the relevance of assumptions made when calculating sample size in randomised controlled trials.

Design Review.

Data sources We searched MEDLINE for all primary reports of two arm parallel group randomised controlled trials of superiority with a single primary outcome published in six high impact factor general medical journals between 1 January 2005 and 31 December 2006. All extra material related to design of trials (other articles, online material, online trial registration) was systematically assessed. Data extracted by use of a standardised form included parameters required for sample size calculation and corresponding data reported in results sections of articles. We checked completeness of reporting of the sample size calculation, systematically replicated the sample size calculation to assess its accuracy, then quantified discrepancies between a priori hypothesised parameters necessary for calculation and a posteriori estimates.

Results Of the 215 selected articles, 10 (5%) did not report any sample size calculation and 92 (43%) did not report all the required parameters. The difference between the sample size reported in the article and the replicated sample size calculation was greater than 10% in 47 (30%) of the 157 reports that gave enough data to recalculate the sample size. The difference between the assumptions for the control group and the observed data was greater than 30% in 31% (n=45) of articles and greater than 50% in 17% (n=24). Only 73 trials (34%) reported all data required to calculate the sample size, had an accurate calculation, and used accurate assumptions for the control group.

Conclusions Sample size calculation is still inadequately reported, often erroneous, and based on assumptions that are frequently inaccurate. Such a situation raises questions about how sample size is calculated in randomised controlled trials.


  • Funding: PC received financial support from Direction Régionale des Affaires Sanitaires et Sociales. AD received financial support from Fédération Hospitalière de France and Assistance Publique des Hôpitaux de Paris. The authors were independent from the funding bodies.

  • Contributors: The guarantor (PR) had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: PC, PR. Acquisition of data: PC, AD. Recalculation of sample sizes: PC, GB. Analysis and interpretation of data: PC, PR, BG, AD, GB. Drafting of manuscript: PC, AD, PR. Critical revision of manuscript for important intellectual content: PR, BG, AD, GB. Statistical analysis: PC, PR, BG, GB, AD. Administrative, technical, or material support: PR. Study supervision: PR, BG.

  • Competing interests: None declared.

  • Ethical approval: Not required.

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