Two-sample t Test
The two-sample t-test is used to test whether two samples come from distributions with the same means. The samples are assumed to come from Gaussian (normal) distributions. If this is not the case, then a nonparametric test, such as the Wilcoxon rank sum test, may be a more appropriate test of location.
This test handles both paired and independent samples.
To perform a two-sample t test
Choose Statistics Compare Samples
Two Samples
t test. The dialog shown below appears.
The two-sample t-test dialog has the following options:
Data
Data Set
Select a data set from the dropdown list or type the name of a data set. You can also type into the Data Set edit field any expression that evaluates to a data set.
Variable 1
Specify a column as the first sample, when the data set does not have a grouping indicator.
Variable 2
Specify another column as the second sample, when the data set does not have a grouping indicator.
Variable 2 is a Grouping Variable
Select if one column in the data set is a grouping indicator that categorizes cases into two groups. In this case, select the response from Variable 1 and the indicator from Variable 2.
Test
Type of t Test
Choose Paired t for a paired t test; choose Two-sample t for an unpaired t test.
Assume Equal Variances
This is enabled only when Two sample t is chosen. Select if the two samples are assumed to come from populations with equal variances.
Hypotheses
Mean Under Null Hypothesis
Enter the difference between the assumed population means of Variable x and Variable y.
Alternative Hypothesis
Specify the alternative hypothesis. For example, to perform a one-sided test against the alternative hypothesis that the mean of Variable 1 is greater than the mean of Variable 2, select greater from the list box.
Confidence Interval
Confidence Level
Enter a number between 0 and 1 to be used as the confidence level.
Results
Enter the name for the object in which to save the results of the analysis.
Print Results
Select this to print out the results of the analysis in the designated output window.
Related S-PLUS language functions
t.test