User Guide

361
TTests
Independent-Samples T Test Options
Figure 20-5
Independent
-Samples T Test Options dialog box
Confidence Interval. By default, a 95%-confidence interval for the difference in means
is displayed. Enter a value between 1 and 99 to request a different confidence level.
Missing Values. When you test several variables and data are missing for one or more
variables, you can tell the procedure which cases to include (or exclude):
 Exclude missing data analysis by analysis. Each t test uses all cases that have valid
data for the variables tested. Sample sizes may vary from test to test.
 Exclude cases listwise. Each t test uses only cases that have valid data for all
variables used in the requested t tests. The sample size is constant across tests.
Paired-Samples T Test
The Paired-Samples T Test procedure compares the means of two variables for a
single group. It computes the differences between values of the two variables for each
case and tests whether the average differs from 0.
Example. In a study on high blood pressure, all patients are measured at the beginning
of the study, given a treatment, and measured again. Thus, each subject has two
measures, often called before and after measures. An alternative design for which
this test is used is a matched-pairs or case-control study. Here, each record in the
data file contains the response for the patient and also for his or her matched control
subject. In a blood pressure study, patients and controls might be matched by age (a
75-year-old patient with a 75-year-old control group member).