Re: Within Ss contrasts vs.pairwise comparison inconsistency

Posted by Bruce Weaver on Aug 01, 2018; 12:45pm
URL: http://spssx-discussion.165.s1.nabble.com/Within-Ss-contrasts-vs-pairwise-comparison-inconsistency-tp5736399p5736414.html

Martin, you replied privately to an email account I don't check regularly--I
use it only for posting to this forum so I don't get a lot of junk in my
everyday inbox.  

See this letter for an explanation about why centering the covariates solves
your problem.

https://pdfs.semanticscholar.org/c0b2/7a4e6b23aaf6c5af20581378d3e2601b11b0.pdf

Van Breukelen GJ, Van Dijk KR. Use of covariates in randomized controlled
trials. Journal of the International Neuropsychological Society. 2007
Sep;13(5):903-4.



Bruce Weaver wrote

> What happens if you use mean-centered versions of your covariates?
>
>
> Here's a demo that may help.
>
> * Get some data for 2x2x(2) model with repeated measures on the last
> factor.
> GET FILE='C:\SPSSdata\survey_sample.sav'.
> * Let A and B be between-Ss factors.
> COMPUTE A = wrkstat EQ 1.
> COMPUTE B = marital EQ 1.
> FORMATS A B (F1).
> DESCRIPTIVES A B.
> * Use the first 2 education variables (educ paeduc) for the repeated
> measures.
> * Use age as the covariate.
> COMPUTE X = age.
>
> SELECT IF NMISS(A,B,X,educ,paeduc) EQ 0.
>
> AGGREGATE
>   /OUTFILE=* MODE=ADDVARIABLES
>   /BREAK=
>   /X_mean=MEAN(X).
> COMPUTE Xmc = X - X_mean.
>
> GLM educ paeduc BY A B WITH X
>   /WSFACTOR=Time 2 Polynomial
>   /EMMEANS=TABLES(Time) WITH(X=MEAN)COMPARE ADJ(LSD)
>   /EMMEANS=TABLES(Time) WITH(X=0)COMPARE ADJ(LSD)
>   /WSDESIGN=Time
>   /DESIGN=X A B A*B.
>
> GLM educ paeduc BY A B WITH Xmc
>   /WSFACTOR=C 2 Polynomial
>   /EMMEANS=TABLES(C) WITH(Xmc=MEAN)COMPARE ADJ(LSD)
>   /WSDESIGN=C
>   /DESIGN=Xmc A B A*B.
>
>
>
>
> msherman wrote
>> Dear List: I am conducting a 2 X 2 X (2) repeated measures (Treatment vs
>> Control,  Younger vs Older,  Time 1 vs Time 2) analysis with 2 covariates
>> (Body Mass Index, duration of illness). Upon examining the repeated
>> measures effect (Tests of within-subjects effects) and the pairwise
>> comparisons results I expected the stats to mirror image each other. That
>> is, if the main effect for Time was not significant then I also expected
>> to see that the pairwise comparison between Time 1 and Time 2 to be not
>> significant. What I find is that the overall Time effect is not
>> significant but the time pairwise comparison output shows that the two
>> times periods are statistically different. The pairwise comparison stats
>> come out under the Estimated Marginal Means evaluated at the means of the
>> two covariates.  Shouldn't the results be the same? What am I missing.
>>
>> Martin F. Sherman, Ph.D.
>> Professor of Psychology
>> Director of Masters Program: Thesis Track
>> Loyola University Maryland
>> 4501 North Charles Street
>> 222 B Beatty Hall
>> Baltimore, MD 21210
>>
>>
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>
>
>
>
>
> -----
> --
> Bruce Weaver

> bweaver@

> http://sites.google.com/a/lakeheadu.ca/bweaver/
>
> "When all else fails, RTFM."
>
> NOTE: My Hotmail account is not monitored regularly.
> To send me an e-mail, please use the address shown above.
>
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-----
--
Bruce Weaver
[hidden email]
http://sites.google.com/a/lakeheadu.ca/bweaver/

"When all else fails, RTFM."

NOTE: My Hotmail account is not monitored regularly.
To send me an e-mail, please use the address shown above.

--
Sent from: http://spssx-discussion.1045642.n5.nabble.com/

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