User Guide

Chapter
20
TTests
Three types of t tests are available:
Independen
t-samples t test (two -sample t test).
Compares the means of one variable
for two groups of cases. Descriptive statistics for each group and Levene’s test for
equality of variances are provided, as well as both equal- and unequal-variance t
values and
a 95%-confidence interval for the difference in means.
Paired-samples t test (dependent t test). Compares the means of two variables for a
single group. This test is also for matched pairs or case-control study designs. The
output inc
ludes descriptive statistics for the test variables, the correlation between
them, descriptive statistics for the paired differences, the t test, and a 95%-confidence
interval.
One-samp
le t test.
Compares the mean of one variable with a known or hypothesized
value. Descriptive statistics for the test variables are displayed along with the t test. A
95%-confidence interval for the difference between the mean of the test variable and
the hypot
hesized test value is part of the default output.
Independent-Samples T Test
The Independent-Samples T Test procedure compares means for two groups of cases.
Ideally
, for this test, the subjects should be randomly assigned to two groups, so that
any difference in response is due to the treatment(orlackoftreatment)andnotto
other factors. This is not the case if you compare average income for males and
female
s. A person is not randomly assigned to be a male or female. In such situations,
you should ensure that differences in other factors are not masking or enhancing a
significant difference in means. Differences in average income may be influenced by
factor
s such as education and not by sex alone.
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