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Validity of sample sizes in publications of randomised controlled trials on the treatment of age-related macular degeneration: cross-sectional evaluation
  1. Sabrina Tulka,
  2. Berit Geis,
  3. Christine Baulig,
  4. Stephanie Knippschild,
  5. Frank Krummenauer
  1. Institute for Medical Biometry and Epidemiology, University Witten Herdecke Faculty of Health, Witten, Germany
  1. Correspondence to Sabrina Tulka; sabrina.tulka{at}


Objective The aim of this cross-sectional study was to examine the completeness and accuracy of the reporting of sample size calculations in randomised controlled trial (RCT) publications on the treatment of age-related macular degeneration (AMD).

Methods A sample of 97 RCTs published between 2004 and 2014 was reviewed for the calculation of their sample size. It was examined whether a (complete) description of the sample size calculation was presented. Furthermore, the sample size was recalculated, whenever possible based on the published details, in order to verify the reported number of patients.

Primary outcome measure The primary endpoint of this cross-sectional investigation was a described sample size calculation that was reproducible, complete and correct (maximum tolerated deviation between reported and replicated sample size ±2 participants per trial arm).

Results A total of 50 publications (52%) did not provide any information on the justification of the number of patients included. Only 17 publications (18%) provided all the necessary parameters for recalculation; 8 of 97 (8%, 95%-CI: 4% to 16%) publications achieved the primary endpoint. The median relative deviation between reported and recalculated sample sizes was 1%, with a range from −43% to +66%.

Conclusion Although a transparent sample size legitimation is a crucial determinant of an RCT’s methodological validity, more than half of the RCT publications considered failed to report them. Furthermore, reported sample size legitimations were often incomplete or incorrect. In summary, clinical authors should pay more attention to the transparent reporting of sample size calculation, and clinical journal reviewers may opt to reproduce reported sample size calculations.

Synopsis More than half of the analysed RCT publications on the treatment of AMD did not report a transparent sample size calculation. Only 8% reported a complete and correct sample size calculation.

  • sample size calculation
  • RCT publication
  • transparent reporting
  • recalculation

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  • Contributors ST extracted the randomised controlled trial publications' relevant outcome data for the sample size calculations (parallel independent evaluation), performed the sample size recalculations and the statistical analysis of the recalculation data, and wrote this systematic review's first draft. BG double-checked the outcome data extraction and assisted in the sample sizes’ evaluation and recalculation; furthermore she reviewed the manuscript draft. CB carried out the publication search, excluded inappropriate articles and revised the manuscript. SK carried out the publication search, excluded inappropriate articles and revised the manuscript. FK wrote the grant application to the Leonhard Stinnes Foundation, designed this investigation, extracted relevant outcome data (parallel independent evaluation) for selected RCT publications, assisted in sample size recalculations, thoroughly revised the first draft of the manuscript and contributed major parts to the second draft version.

  • Funding This work was supported by Leonard Stinnes Foundation grant number, internal reference, KS 11535.

  • Competing interests This systematic review was conducted by ST (MSc Statistics) within the framework of a full-time placement as a research assistant. This placement was funded by a 24-month research grant received from the Leonard Stinnes Foundation (internal reference: KS 11535). The study has no political conflict of interest, neither in terms of content nor with regard to the results. The results presented in this manuscript are part of the doctoral thesis of ST to be submitted to the Faculty of Health of Witten/Herdecke University to achieve the doctoral degree 'Dr rer medic' (Doctor of Theoretical Medicine). Furthermore, the results contained in this article have already been presented by means of an oral presentation at the annual meeting of the German Region of the International Biometric Society (Frankfurt/Main, Germany, March 2018) and by means of a poster presentation at the annual meeting of the German Ophthalmic Surgeons (Nuremberg, Germany, June 2018), where the presentation was awarded with the 2018 poster prize of the Ophthalmic Surgeons.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement Data are available upon reasonable request.

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