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SPSS
T-Test
You have 3 outputs to send in this week. First, output associated w/ Attitude Toward Staff;
output associated w/ pre-test & post-test depression scores & output associated w/ Attitude
Toward Drinking & Driving for Controlled & Experimental groups.
When you first open up your SPSS screen, make sure you click on the “variable view” tab
at the bottom so you can define your variables by typing in a single word under the
“Name” column & then you can better define the variable under the “label” column.
Remember to save both your data & then the output screens to be turned in.
Step 1:
Plug in the following data into SPSS: Under the “name” column, you may list
“staff” & then under the “label” column, you can type in “attitude toward staff”
Attitude Toward Staff
_______________________________________
2
4
1
10
5
3
6
1
4
0
1
4
6
5
6
4
7
1
3
1
_______________________________________
Step 2:
Save your data as “Attitude toward Staff”
Step 3:
Click on “Analyze” at the top of the screen, then on “Compare Means” on the
drop down menu & then click on “One Sample T Test” off the Compare Means drop
down menu, which allows you to evaluate one set of data using a T-Test. The purpose of
a T-test for a single sample mean is to determine whether the mean for a random sample
of individuals differs from a known value or a hypothetical value.
Step 4:
Once the new screen comes up, you will move “Attitude Toward Math” under
“Test Variable(s)” by clicking on the arrow (►) button & then click on “OK.”
Step 5:
The outcome you will see will provide you with a number of different values. The
first box will give you basic descriptive data (the ‘N’ value, mean, standard deviation,
etc…). The second box will then give you the values associated with the T-test
calculations. The very first value given is the t-test value, listed under the column “t.”
Following that value is the degrees of freedom, significance level (which also indicates
whether or not you are using a one-tailed or two-tailed test) among other values. The
main outcome value, or the T-test value is that value listed under the ‘t’ column in this
output.
Step 6:
Save this output as “Attitude toward math t-test” & send to me as an attachment
to an email for this week’s assignment.
Paired Samples T-test
Step 1:
Follow the steps to input the following data into the SPSS program (Don’t forget
to name your variables under the variable view screen!!):
Pretest & Posttest Depression Scores
_____________________________________________________
Subject #
Pretest score
Posttest score
1
10
9
2
12
9
3
9
10
4
15
15
5
11
8
6
14
10
7
8
9
8
13
10
9
12
7
10
7
6
___________________________________________________
Step 2:
Save the data as “Pretest & Posttest Depression Scores.” Always remember to
double check your data for accuracy after you have plugged it in.
Step 3:
Click on “Analyze” at the top of the screen followed by “Compare Means” & then
“Paired-Samples T Test.”
Step 4:
The next screen will allow you to move over (►) the “pretest” data under the
column entitled “Variable 1” & then the “posttest” data under the “variable 2” column.
Then click “OK” at the bottom of the screen.
Step 5:
The output here again offers basic descriptive values associated with your data.
The T-test value for this example is located in the last box, 3’rd column from the end.
Step 6:
Save this output as “Pretest & Posttest Depression Output” & send to me as a part
of this week’s assignment.
Independent Samples T-Test
Step 1:
Follow the steps for inputting data into the system. Entitle your first column as
“attitude” & type in “Attitudes toward drinking & driving” for your label. The second
column, simply entitle “groups” giving a value of “1” for the experimental group & “2”
for the control group.
Attitudes Toward Drinking & Driving for Experimental & Control Groups
___________________________________________________
Subject #
Group codes / group
Attitudes toward drinking & driving
1
1 (Experimental)
12
2
2 (Control)
14
3
2 (Control)
14
4
1 (Experimental)
10
5
1 (Experimental)
12
6
2 (Control)
15
7
1 (Experimental)
8
8
2 (Control)
16
9
1 (Experimental)
12
10
2 (Control)
10
11
1 (Experimental)
9
12
1 (Experimental)
11
_____________________________________________________________________
Step 2:
Remember to save your data as “Attitudes Toward Drinking & Driving” & double
check your data.
Step 3:
Go to the top of the screen & click on “Analyze,” then “Compare Means” &
“Independent Samples T-Test.” Move “Attitude” scores over to the “Test Variable(s) box
& then “Group” over to the “Grouping Variable” box.
Step 4:
The program will want you to “define the groups” by simply clicking on the
“Define Groups” button. A small screen will appear entitled “Use Specific Values.” It
may seem a bit redundant, but next to “Group 1” type in the number “1” & next to
“Group 2” type in the number “2.”
Step 5:
In that same window, you will see the “options” button next to the “test variables”
column. Click on the “options” button. SPSS automatically sets the alpha level at .05 (as
you see, it lists the confidence interval as 95%). To change this to an alpha level of .01,
change the “confidence interval percentage” to 99% & then click “continue.”
Step 6:
Once you have clicked continue & you are back to your original window, click
“OK” at the bottom of the window.
Step 5:
Your t-value for this function will appear in the third column from the left under
the “t” column in the second box on your screen after the usual descriptive information.
Step 6:
Save the output & send it to me as a part of this week’s assignment.
It can sometimes be hard to find the final value that you are looking for in these outputs
due to the many other values that are present on the screen. Some of the values are basic &
recognizable, such as the ‘N’ value, the mean & standard deviation values, etc.. But there
are also other values present such as “Standard Error Difference” & “Significance” value
(the column where it indicates whether or not you are working a one-tailed or two-tailed
test). Not all of the values will be of significance to you at this point. There is one step that I
would like to point out. It is in regard to the last column to the right in your output that
reports on a “95% confidence interval.” Remember the alpha levels utilized in hypothesis
testing?
From your notes on hypothesis testing: (α) A specific probability value, known as the level
of significance or alpha level that sets the boundaries of high probability samples vs. low
probability samples. By convention, commonly used alpha levels are α = .05 (5%); α = .01
(1%), & .001 (0.1%). Critical region: Extremely unlikely values in the tail regions of the
distributions that are inconsistent w/ the null hypothesis (that is, they are unlikely to occur
if the null hypothesis is true). The boundaries for the critical region are determined by the
alpha level. If sample data fall in the critical region, the null hypothesis is rejected.
The SPSS system is automatically set at .05 alpha level, or 5%, leaving 95% of the rest of
the distribution or null region as we did in the problem above. You can change the alpha
level & thus the rejection & null region of the distribution in the program. Once you have
clicked on “Independent Samples T Test,” on the screen where you move your variables
over to the “Test Variable(s) & the Groups over to the “Grouping Variable,” you will see
an “Options” button to the right of the screen. Click on the “Options” button & this is
where you can change the 95% to 99 (associated with the .01 alpha level) or 99.9 (associated
with the .001 alpha level).