Wemedoo Sample Size and Power Calculator

TEST FOR TWO PROPORTIONS

Purpose: To test for the number of subjects required to detect a significant difference between two sample proportions.

Example: The aim of the study is to examine a combination of drugs that decrease the percentage of adverse events. The standard of care drug therapy induces adverse events in 18% of patients, while the assumption is that a new combination of drugs will lower the percentage of adverse events to 9%. The significance level, or alpha (α), is set to 0.05 (probability of obtaining false positive result in statistical testing).

SAMPLE SIZE CALCULATION

How many subjects are sufficient to obtain a significant difference in the proportions of adverse events between the study groups (experimental vs. standard of care) using a power of 80%, and an allocation ratio of 1:1 (equal sample sizes)?

Design:
Alpha (α) = 0.05
Power = 0.80
Proportion 1 = 0.18
Proportion 2 = 0.09
Allocation Ratio (N2/N1): 1

Result: A sample size of 226 subjects per group achieves an 80% power to detect a significant difference in the proportions of adverse events between the two groups.

POWER CALCULATION

What power is achieved by enrolling 280 subjects per group in the same study with the same proportions (18% vs. 9%)?

Design:
Alpha (α) = 0.05
Sample Size 1 (N1) = 280
Sample Size 2 (N2) = 280
Proportion 1 = 0.18
Proportion 2 = 0.09

Result: A sample size of 280 subjects per group achieves an 88% power to detect a significant difference between the groups examined with regards to the proportion of adverse events.