Massachusetts Institute of Technology
Department of Urban Studies and Planning

11.220 Quantitative Reasoning and Statistical Methods for Planning

SPSS

11.220-HOME   |   TEST-OUT   |   BRUSH-UP   |   Brushup-DATA

SPSS > Independent-samples T-test

1. If we want to compare the mean scores of two groups on a given variable we could use the "Independent-Samples T-Test." We test the Null Hypothesis that the two means are equal.

2. Go to Analyze->Compare Means and click "Independent-Samples T-Test." Choose "incws" and "educ_yr" as the "Test Variables:."

3. Select "male" as the "Grouping Variable:" as in the above window. Then click "Define Groups..." and set "Group 1:" as 0 (female) and "Group 2:" as 1 (male).

4. The Levene's Test for Equality of Variances tells us if we have met our second assumption, i.e., the two groups have approximately equal variance for these two variables. If the Levene's Test is significant (the value under "Sig." is less than .05), the two variances are significantly different. If it is not significant (Sig. is greater than .05), it means the two variances are approximately equal. If the Levene's test is not significant, we have met our second assumption. Here, we can assume that the variances of these two variables between the two groups is approximately equal.

Since the variances are equal we read the top line for each variable tested when interpreting the t-test. The t-test Sig. value for incws is .044, which is less than .05. Therefore, we can conclude that the means for income of the two groups are significantly different. The t-test Sig. value for educ_yr is .402, and is much greater than .05. So we conclude that the means of the two groups, when it comes to years of education, are not sigificantly different.

We set females as Group 1 and males as Group 2 and our Null Hypothesis is m1-m2=0. The mean difference reported above is -$12,288.03, meaning women are earning less. Also, we are 95% confident that the mean annual salary of women in the population from which this sample was drawn is between -$24,200.60 and -$375.421 less than men.

Created by Myounggu Kang on January 25, 2004. Edited by Rhonda Ryznar on January 20, 2005.