<Table> <Tr> <Td> </Td> <Td> This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations . Please help to improve this article by introducing more precise citations . (November 2009) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations . Please help to improve this article by introducing more precise citations . (November 2009) (Learn how and when to remove this template message) </Td> </Tr> <P> A Z - test is any statistical test for which the distribution of the test statistic under the null hypothesis can be approximated by a normal distribution . Because of the central limit theorem, many test statistics are approximately normally distributed for large samples . For each significance level, the Z - test has a single critical value (for example, 1.96 for 5% two tailed) which makes it more convenient than the Student's t - test which has separate critical values for each sample size . Therefore, many statistical tests can be conveniently performed as approximate Z - tests if the sample size is large or the population variance is known . If the population variance is unknown (and therefore has to be estimated from the sample itself) and the sample size is not large (n <30), the Student's t - test may be more appropriate . </P>

Which of the following is a requirement for using the z-test for differences of proportions