How to reduce the number of patients needed for randomized trials: A basic introduction Academic Article uri icon


MeSH Major

  • Randomized Controlled Trials as Topic
  • Sample Size


  • Complementary therapies are often used to treat patients with persisting conditions. In a typical randomized trial, patients are assessed for symptom severity at baseline, randomized to treatment or control and then reassessed after a suitable follow-up period. It can be shown that the number of patients required for such trials can be reduced by the use of two methods: repeat measurement and analysis of co-variance (ANCOVA). An example is given of a trial of acupuncture for back pain. Administering the pain questionnaire twice at baseline and four times at follow-up reduces the number of patients by approximately 20% compared to single questionnaire administration. Use of ANCOVA decreases the number of patients by approximately a further 10%. There are considerable economic and ethical advantages to reducing sample size by a third; however, ANCOVA is statistically complex and expert statistical help is required.

publication date

  • December 2001



  • Academic Article



  • eng

Digital Object Identifier (DOI)

  • 10.1054/ctim.2001.0467

PubMed ID

  • 12184352

Additional Document Info

start page

  • 234

end page

  • 6


  • 9


  • 4