SPSS Activities
13 TABLE
SPSS ACTIVITY
ANCOVA
13.4
Data set.
DRUG 1
DRUG 2
Before
After
Before
After
5
20
7
19
10
23
12
26
12
30
27
33
9
25
24
35
23
34
18
30
21
40
22
31
14
27
26
34
18
38
21
28
6
34
14
23
13
31
9
22
Q
Twenty male volunteers who are overweight were randomly placed into two groups of 10 subjects each. The purpose of the study was to determine whether a difference existed in the mood of these individuals after they ingested one of two different diet drugs. The concern was that these drugs might cause mood changes as a side-effect. The subjects’ mood was measured with a five-item checklist, which was administered just prior to administration of the drug and again two hours later. The higher the score on the checklist, the higher the subject’s positive mood. Table 13.4 gives the data collected.
Procedures and Questions A. Examine the data, and then enter the data into SPSS with
three columns: Drug, Before, and After. First, run a one-way ANOVA, using the “Before” scores as the dependent variable, to see whether the two groups differed on mood scores before ingesting the drug, even though they were assigned to the groups randomly.
1. What is the overall mood mean for all of the subjects? 2. What are the Before mood means for the two groups? 3. Are the two means significantly different at = .05?
B. What the researcher really wants to know is whether there is a difference in
mood after administration of the drug, so now run a one-way ANOVA, using the “After” scores as the dependent variable. Q
4. What is the overall mood mean for all subjects? 5. What happened to the overall average mood of the subjects after administration of the drugs? 6. What are the After mood means for the two groups? 7. Are the two mood means significantly different at = .05? 8. How many degrees of freedom does the error term have in this ANOVA? 9. On the basis of these two ANOVAs, is it reasonable to conclude that either drug results in an increase in positive mood and that the two drugs do not differ significantly in the level of improvement in positive mood?
C. Being a perceptive statistics student and a statistical wizard, you realize
that the two groups in this study are somewhat different (although not statistically significantly) on the initial measurement of mood. You decide that an analysis of covariance (ANCOVA) is in order. Q 10. In effect, what does the ANCOVA do to each subject’s “After” score? D. To run the ANCOVA:
Go to Analyze→General Linear Model→Univariate.
n
n
n
n
n
Enter “Drug” in the Fixed Factor box, “After” in the Dependent Variable box, and “Before” in the Covariate box. Click on Options and move (OVERALL) and Drug into the Display means for: box. Check Descriptive statistics. Click Continue and OK. Use the Tests of Between-Subjects Effects box to answer the following questions:
Q 11. What is the F-ratio for the drugs?
12. Is it significant at = .05?
13. How many degrees of freedom does the error term have in the ANCOVA?
14. Why did the degrees of freedom change from the one-way ANOVA?
E. Go to the Estimated Marginal Means Grand Mean box.
Q 15. What is the grand mean?
16. How does this compare to your answer to question 4?
17. Why didn’t the grand mean change?
Go to box 2 under Estimated Marginal Means for “Drug.” Q 18. What are the mood means for the two groups?
19. How do they compare to your answer to question 6?
20. What has happened to the adjusted average after mood score for the subjects taking drug 1?
21. What has happened to the adjusted average after mood score for the subjects taking drug 2?
22. Why were these adjustments made?
23. On the basis of the ANCOVA and the information available to you, which drug should the company manufacture?
SPSS ACTIVITY
ANCOVA
13.4
Data set.
DRUG 1
DRUG 2
Before
After
Before
After
5
20
7
19
10
23
12
26
12
30
27
33
9
25
24
35
23
34
18
30
21
40
22
31
14
27
26
34
18
38
21
28
6
34
14
23
13
31
9
22
Q
Twenty male volunteers who are overweight were randomly placed into two groups of 10 subjects each. The purpose of the study was to determine whether a difference existed in the mood of these individuals after they ingested one of two different diet drugs. The concern was that these drugs might cause mood changes as a side-effect. The subjects’ mood was measured with a five-item checklist, which was administered just prior to administration of the drug and again two hours later. The higher the score on the checklist, the higher the subject’s positive mood. Table 13.4 gives the data collected.
Procedures and Questions A. Examine the data, and then enter the data into SPSS with
three columns: Drug, Before, and After. First, run a one-way ANOVA, using the “Before” scores as the dependent variable, to see whether the two groups differed on mood scores before ingesting the drug, even though they were assigned to the groups randomly.
1. What is the overall mood mean for all of the subjects? 2. What are the Before mood means for the two groups? 3. Are the two means significantly different at = .05?
B. What the researcher really wants to know is whether there is a difference in
mood after administration of the drug, so now run a one-way ANOVA, using the “After” scores as the dependent variable. Q
4. What is the overall mood mean for all subjects? 5. What happened to the overall average mood of the subjects after administration of the drugs? 6. What are the After mood means for the two groups? 7. Are the two mood means significantly different at = .05? 8. How many degrees of freedom does the error term have in this ANOVA? 9. On the basis of these two ANOVAs, is it reasonable to conclude that either drug results in an increase in positive mood and that the two drugs do not differ significantly in the level of improvement in positive mood?
C. Being a perceptive statistics student and a statistical wizard, you realize
that the two groups in this study are somewhat different (although not statistically significantly) on the initial measurement of mood. You decide that an analysis of covariance (ANCOVA) is in order. Q 10. In effect, what does the ANCOVA do to each subject’s “After” score? D. To run the ANCOVA:
Go to Analyze→General Linear Model→Univariate.
n
n
n
n
n
Enter “Drug” in the Fixed Factor box, “After” in the Dependent Variable box, and “Before” in the Covariate box. Click on Options and move (OVERALL) and Drug into the Display means for: box. Check Descriptive statistics. Click Continue and OK. Use the Tests of Between-Subjects Effects box to answer the following questions:
Q 11. What is the F-ratio for the drugs?
12. Is it significant at = .05?
13. How many degrees of freedom does the error term have in the ANCOVA?
14. Why did the degrees of freedom change from the one-way ANOVA?
E. Go to the Estimated Marginal Means Grand Mean box.
Q 15. What is the grand mean?
16. How does this compare to your answer to question 4?
17. Why didn’t the grand mean change?
Go to box 2 under Estimated Marginal Means for “Drug.” Q 18. What are the mood means for the two groups?
19. How do they compare to your answer to question 6?
20. What has happened to the adjusted average after mood score for the subjects taking drug 1?
21. What has happened to the adjusted average after mood score for the subjects taking drug 2?
22. Why were these adjustments made?
23. On the basis of the ANCOVA and the information available to you, which drug should the company manufacture?
