z-Test

Abstract

Many research questions involve testing differences between two population proportions (percentages). For example, Is there a significant difference between the proportion of girls and boys who smoke cigarettes in high school?, Is there a significant difference in the proportion of foreign and domestic automobile sales?, or Is there a significant difference in the proportion of girls and boys passing the Iowa Test of Basic Skills? These research questions involve testing the differences in population proportions between two independent groups. Other types of research questions can involve differences in population proportions between related or dependent groups. For example, Is there a significant difference in the proportion of adults smoking cigarettes before and after attending a stop smoking clinic?, Is there a significant difference in the proportion of foreign automobiles sold in the U.S. between years 1999 and 2000?, or Is there a significant difference in the proportion of girls passing the Iowa Test of Basic Skills between the years 1980 and 1990? Research questions involving differences in independent and dependent population proportions can be tested using a z-test statistic. Unfortunately, these types of tests are not available in most statistical packages, and therefore you will need to use a calculator or spreadsheet program to conduct the test.