Ψ420
Ainsworth
Lab 8 – One-way ANCOVA
A drug company is interested in testing which medicine is best at relieving
headaches. They randomly selected 15 headache suffers and randomly assigned them to
take Tylenol, Advil or Excedrin. Each subject was asked to rate the severity of their
headaches before and after taking the medication.
Tylenol
Pre
Post
8
9
2
6
7
9
8
9
7
6
Advil
Pre
Post
8
9
8
9
7
8
8
9
8
9
Excedrin
Pre
Post
4
7
3
2
6
5
2
1
6
1
1. Run a one-way ANOVA on the post-test data (ignoring pre-test). You’ll need to
reorganize the data.
a. Click on options and include, descriptives, effect size, homogeneity tests,
and move drug over to “display means for”.
b. Click on post hoc and include a Tukey test for drug.
2. Run an ANCOVA using pretest as a covariate.
a. Go to Analyze -> GLM -> Univariate.
b. Move post into DV and drug into fixed effects.
c. Move pre into covariate.
d. Click on options and include, descriptives, effect size, homogeneity tests,
and move drug over to “display means for”.
e. Click on post hoc and include a Tukey test for drug.
3. Annotate the output; make notes on how the output changed from the ANOVA
run and write a results section for the ANCOVA run.
4. Rerun the ANCOVA using the traditional approach, by hand.