To effectively compute sample size, we may require information about your study as well previous studies. Below is an outline of the necessary information followed by a brief example.
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Express research hypothesis as a statistical (null) hypothesis
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Determine whether alternative hypothesis is one or two directional
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Specify level of significance (e.g., 0.05)
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Specify estimate of standard deviation of variables to be analyzed
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Specify estimates of group means (effect size)**
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Specify desired power (e.g., 80%) **
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Determine sample size per group **
Example
A clinical trial is conducted to investigate the efficacy of treatment A versus treatment B for glucose control. The primary outcome variable is A1C at 6 months. The statistical hypotheses are given below.
| Null Hypothesis: Mean A1C the same for two treatment patient 'Populations' | |
| Alternative: Means not the same |
Parameter estimates were based on a small sample of diabetic patients given treatment A. Their mean A1C is 8.4 with standard deviation 2.3. The investigator wants p-values equal to or less than 0.05 to reject the null hypothesis and power at least 80% to reject if mean for B is 1 or more units different from 8.4.
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Effect size |
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The required sample size is 85 for each of the two treatment groups for 170 total patients.