Re: Repeated measures ANOVA with one measurement
- From: Aniko <aniko123_57@xxxxxxxxx>
- Date: Mon, 16 Jul 2007 18:44:23 -0000
A few follow up questions and
clarifications:
First, the real setup.
- I have 10 subjects (7 good performers, 3 bad performers - hence the
suggestion that I do a repeated-measures ANOVA to remove some of the
variance between subjects)
- I have three factors: 4 M's, 4 N's, and 4 S's. Thus 64 conditions.
- I measure percent correct (Pc - transformed to a linear scale) once
for each condition.
So, from your response, it appears that all the 'yes'es apply.
- Lastly, I'd like to use Tukey post-hoc tests and I heard that in an
ANCOVA the power of post-hoc tests are dramatically reduced.
Now the questions:
1) Are the power of post-hoc tests reduced in RM ANOVA like an ANCOVA?
I am not sure what have you heard about ANCOVA. The power is reduced
compared to what? Are there any other "legal" alternatives? In
general, having unaccounted for variation reduces power, so repeated
measures ANOVA should do better than regular ANOVA, not to mention not
violating the independence assumption.
2) Since a 1-way RM ANOVA is equivalent to a 2-way ANOVA without
interaction, does this mean that a 2-way RM ANOVA is equivalent to a 3-
way ANOVA without interactions? I'd like to be able to check the
significance of the interaction between N and M, N and S, and M and S.
Don't worry, you can have interactions between your factors of
interest (N,M,S), but they cannot interact with the nuisance repeated
measure factor - subject. So a 3-way repeated measures ANOVA is the
same as a 4-way ANOVA with no interaction with the repeated measure
factor.
3) Technical SPSS v 15 question: How do I do this RM ANOVA with one
measurement?
- I go to Analyze > General Linear Model > Repeated Measures ...
- A text box opens asking for a "within-subject factor name" and
number of levels, and a "measure name". The first is necessary, the
second seems to be an optional label. It seems to me that I'd want
"Pc" to be my within-subject factor name and my number of levels to be
1. However, it won't let me have just 1 level, at least 2 are
necessary for this program. (This I think is my major problem. It
makes me think SPSS doesn't do 1 measurement RM ANOVA.) Then as we go
to the next screen it allows me to select my within-subject variables
(numbers only allowed, so Pc) and the between-subject factors (M, N,
S).
Again, I have not used SPSS, so I might be wrong, but I think you have
not set up a repeated measure factor - Subject. This should have a
unique value for each subject. Then you would have 10 levels for your
repeated measures factor. "Pc" is not a factor, but the outcome. The
data might look like this:
Subject M N S Pc
1 M1 N1 S1 0.7
1 M2 N1 S1 0.8
1 M1 N2 S1 0.4
....
10 M2 N2 S2 0.6
Anyway, that's how other software tends to like it.
4) I thought of a way around my problem by duplicating the Pc scores,
thus having Pc1 and Pc2. So I could have 2 levels for SPSS. Then the
RM ANOVA in SPSS would probably work. But I'm guessing this would
artificially inflate my error terms. Is this a good idea or not?
No, it certainly does not sound like a good idea.
Just as a side-note: if your outcome is often close to 100% or 0%, you
might have all kind of issues with the basic ANOVA assumptions of
additivity and equal variance.
Aniko
.
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