Reporting of sample size calculation in randomised controlled trials: review

doi:10.1136/bmj.b1732

Published 12 May 2009, doi:10.1136/bmj.b1732
Cite this as: BMJ 2009;338:b1732

Research

Reporting of sample size calculation in randomised controlled trials: review

Pierre Charles, research fellow in epidemiology, specialist registrar in internal medicine¹^,2^,3, Bruno Giraudeau, assistant professor of statistics¹^,4^,5^,6, Agnes Dechartres, research fellow in epidemiology¹^,2^,3, Gabriel Baron, statistician¹^,2^,3, Philippe Ravaud, professor of epidemiology¹^,2^,3

¹ INSERM, U738, Paris, France, ² Université Paris 7 Denis Diderot, UFR de Médecine, Paris, ³ AP-HP, Hôpital Bichat, Département d’Epidémiologie, Biostatistique et Recherche Clinique, Paris, ⁴ INSERM Centre d’Investigation Clinique 202, Tours, France, ⁵ Université François Rabelais, Tours, ⁶ CHRU de Tours, Tours

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}bch.aphp.fr

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

Design Review.

Data sources We searched MEDLINE for all primary reports oftwo arm parallel group randomised controlled trials of superioritywith a single primary outcome published in six high impact factorgeneral medical journals between 1 January 2005 and 31 December2006. All extra material related to design of trials (otherarticles, online material, online trial registration) was systematicallyassessed. Data extracted by use of a standardised form includedparameters required for sample size calculation and correspondingdata reported in results sections of articles. We checked completenessof reporting of the sample size calculation, systematicallyreplicated the sample size calculation to assess its accuracy,then quantified discrepancies between a priori hypothesisedparameters necessary for calculation and a posteriori estimates.

Results Of the 215 selected articles, 10 (5%) did not reportany sample size calculation and 92 (43%) did not report allthe required parameters. The difference between the sample sizereported in the article and the replicated sample size calculationwas greater than 10% in 47 (30%) of the 157 reports that gaveenough data to recalculate the sample size. The difference betweenthe assumptions for the control group and the observed datawas greater than 30% in 31% (n=45) of articles and greater than50% in 17% (n=24). Only 73 trials (34%) reported all data requiredto calculate the sample size, had an accurate calculation, andused accurate assumptions for the control group.

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

© Charles et al 2009
This is an open-access article distributed under the terms of the Creative Commons Attribution Non-commercial License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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