Independent Sample T-Test
These instructions assume you have entered
your data into the SPSS spreadsheet and you have selected a grouping (independent)
variable and a test (dependent) variable to analyze. For this example, the variables
gender and achieve are being used. As you use these instructions, you need to substitute
whatever grouping and dependent variable you are using.
- A t-test is used to compare the means of two groups. An Independent
Sample T-Test is used with groups which do not share pairs of scores. Move the cursor to
Statistic on the top menu bar and press the mouse button once.
- Move the cursor to Compare Means and press the mouse button once.
- Move the cursor to Independent Sample T-Test... and press the mouse
button once.
- Since you are comparing the means of two groups, you need to instruct
the computer what variable is being used to identify group members. For this example males
and females will be compared and gender will be the grouping variable. Move the cursor to
Gender on the variable list that appears in the box on the left side of the screen and
click once. This highlights Gender. Move the cursor to the > next to the Grouping
Variable and click on the >.
- Gender should now be listed under Grouping Variable. The grouping
variable is commonly known as the independent variable and it is a categorical variable.
You will also notice some ?,? . The program needs to know what two groups you wish to
compare. You might have selected a Grouping Variable that had more than two groups. Since
T-Tests compare only two groups, you have the option of selecting which two groups. Move
the cursor to the Define Groups... button and click the mouse button.
- A Define Groups... dialog box will appear. Move the cursor to the box
after Group 1 and click the mouse button once. Type 1. Move the cursor to the box after
Group 2 and click the mouse button once. Type 2. You have indicated that the two groups
you wish to use are labeled 1 and 2. In this case they are males and females. For example,
if you had used a different grouping variable and there were five groups, you might have
indicated 2 and 5 if you wished to compare the means of individuals from groups 2 and 5.
When you are finished, you click on Continue.
- You now need to identify the Test Variable (also known as the
dependent variable. This should be the continuous variable on which you wish to compare
the two groups. For example, you might wish to use achievement. Move the cursor to Achieve
in the variable list and press the mouse button once. This highlights the Achieve
variable.
- Move the cursor to the > by Test Variable(s): box and press the
mouse button once. This moves the Achieve variable into the Test Variable(s):
identification box.
- Because you have selected your Grouping Variable (also known as the
independent variable) and your Test Variable (also known as the dependent variable), you
are now ready to run your analysis. Move the cursor to OK and press the mouse button once.
The Output should show the results of the analysis within a few seconds.
Del Siegle
dsiegle@uconn.edu