# Why should I use a 2 sample Poisson rate test?

To perform this test, select Stat > Basic Statistics > 2 Sample Poisson Rate.

Use this analysis to:
• Determine whether the rates of occurrence for two groups differ.
• Calculate a range of values that is likely to include the difference between the population rates.

For example, a traffic safety advocate claims that Elm Street, which has no sidewalks, has more traffic than nearby Main Street, which does have sidewalks. To prove their assertion, each weekday for a month the safety advocate counts the number of cars that travel on each street. These counts follow Poisson distributions, so they compare the amount of traffic on each street with a 2-sample Poisson rate test.

For 2-Sample Poisson Rate, the hypotheses are:
Null hypothesis
 H0: λ = λ0 The difference between the population rates of two samples (λ) equals the hypothesized difference (λ0).
Alternative hypothesis
Choose one:
 H1: λ ≠ λ0 The difference between the population rates of two samples (λ) is not equal to the hypothesized difference (λ0). H1: λ > λ0 The difference between the population rates of two samples (λ) is greater than the hypothesized difference (λ0). H1: λ < λ0 The difference between the population rates of two samples (λ) is less than the hypothesized difference (λ0).
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