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.

Author’s response:

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.


Send your comments to fpmedit@aafp.org. Submission of a letter will be construed as granting AAFP permission to publish the letter in any of its publications in any form. We cannot respond to all letters we receive. Those chosen for publication will be edited for length and style.


Copyright © 2004 by the American Academy of Family Physicians.
This content is owned by the AAFP. A person viewing it online may make one printout of the material and may use that printout only for his or her personal, non-commercial reference. This material may not otherwise be downloaded, copied, printed, stored, transmitted or reproduced in any medium, whether now known or later invented, except as authorized in writing by the AAFP. Contact fpmserv@aafp.org for copyright questions and/or permission requests.

Want to use this article elsewhere? Get Permissions


May-Jun 2019

Access the latest issue
of FPM journal

Read the Issue

FPM E-Newsletter

Sign up to receive FPM's free, weekly e-newsletter, "Quick Tips & Insights."

Sign Up Now


Improving Influenza Vaccination Rates

Here's what you need to know to increase the influenza vaccination rate in your practice.