Use Power and Sample Size for 2-Sample Poisson Rate to examine the relationship between power, sample size, and the comparison rate when you want to compare the difference between two population rates. These calculations require that your data are counts per unit. Use these calculations for the following reasons:

- Before you collect data for a 2-sample Poisson rate test, to ensure that your test has an adequate sample size to achieve acceptable power
- After a 2-sample Poisson rate test, to improve the design for the next test

For example, a traffic safety consultant wants to compare the number of cars per hour that travel on two different streets. Before collecting the data for a 2-sample Poisson rate test, the consultant uses a power and sample size calculation to determine how large the sample must be to obtain a power of 80% (0.8).

To perform a power and sample size calculation for a 2-sample Poisson rate test, choose

.If your data classify each observation into one of two categories, such as pass/fail, use Power and Sample Size for 2 Proportions. For more information on data types, go to Data types you can analyze with a hypothesis test.