Question 662:

Asked on March 23, 2009

Tags: 2-proportion test

1

Answer:

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1. Q662_1_2proportions.xls

The statistical test you are performing is called the 2-proportion test. This test uses the binomial distribution. However, when the sample size in both groups is sufficiently large and close to .5, you can use the normal approximation to the binomial. The general rule is to use the normal approximation (z-score) when np > 5 and nq >5 where n is the sample size and p is the proportion and q is 1-p.

I've put together a calculator (see the attached file) which will conduct the 2-proportion test for you using the normal approximation to the binomial and generate the z-score. It also provides you with the 2-sided p-value associated with the z-score letting you know if the value is significant. The common default value for significance is if the p-value is less than .05.