Determining a proper sample size
Fam Pract Manag. 2004 Oct;11(9):14.
To the Editor:
In the article “A Simple Method for Evaluating the Clinical Literature” [May 2004, page 47], Dr. Robert Flaherty suggests that 400 subjects is a reliable sample size for a study to have adequate statistical power. Using this approach, I recently evaluated the first 40 abstracts for the Journal of the American Medical Association that I found by searching Medline for “randomized controlled trials.” Of the 27 empirical studies, 52 percent had sample sizes less than 400. Dr. Flaherty’s approach would suggest ignoring the evidence from more than half of the randomized controlled trials published in JAMA.
My recommendation of a sample size of at least 400 refers to a sample size above which the power of a study can be safely assumed. I agree with Dr. Fogel that, depending on the study, a smaller sample size may prove adequate for valid statistical analysis. The key point of my article is that physicians need quick and easy “rules of thumb” which, while not perfect, allow reasonably effective evaluation of journal articles. The “Rule of 400” is just such an imperfect rule, yet it is still useful. Nevertheless, I would enthusiastically welcome an easily applied, more accurate guideline for sample size.
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