Sample size calculation with simple math for clinical researchers.

Abstract

Clinicians often need a quick and rough idea of the sample size to assess the feasibility of their clinical research question, but developing countries often lack access to online calculators or its language. I describe a formula that clinicians, residents or any health researcher can remember and use to calculate sample size with mental arithmetic or with the use of a simple pocket calculator. This article covers controlled clinical trials. The formula for two equal-sized groups is simple: n = (16p [100-p])/d2 per group for dichotomous outcomes, where p is average of the two proportions with events, and d is the difference between the two proportions. For continuous scale outcomes, the formula is n=16s2/d2 per group where s is the standard deviation of the outcome data and d is the difference to be detected. The formula needs to be modified for unequal-sized groups. This simple formula may be helpful to clinicians, residents and clinical researchers to calculate sample size for their research questions. The feasibility of many research questions can be easily checked with the calculated sample size.