From: Larry A Sonna <larry_sonna_at_hotmail.com>

Date: Sat 10 Sep 2005 - 00:10:06 EST

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Sep 10 07:27:26 2005

Date: Sat 10 Sep 2005 - 00:10:06 EST

Dear R community,

I am trying to resolve a discrepancy between the way SPSS and R handle 2-way, repeated measures ANOVA.

An experiment was performed in which samples were drawn before and after treatment of four groups of subjects (control and disease states 1, 2 and 3). Each group contained five subjects. An experimental measurement was performed on each sample to yield a "signal". The before and after treatment signals for each subject were treated as repeated measures. We desire to obtain P values for disease state ("CONDITION"), and the interaction between signal over time and disease state ("CONDITION*TIME").

Using SPSS, the following output was obtained:

DF SumSq (Type 3) Mean Sq F value P= COND 3 42861 14287 3.6450.0355

TIME 1 473 473 0.1750.681

COND*TIME 3 975 325 0.1200.947

Error 16 43219 2701

By contrast, using the following R command:

summary(aov(SIGNAL~(COND+TIME+COND*TIME)+Error(EXPNO/COND), Type="III"))

the output was as follows:

Df Sum Sq Mean Sq F value Pr(>F) COND 3 26516 8839 3.2517 0.03651 * TIME 1 473 473 0.1739 0.67986 COND:TIME 3 975 325 0.1195 0.94785 Residuals 28 76107 2718

I don't understand why the two results are discrepant. In particular, I'm not sure why R is yielding 28 DF for the residuals whereas SPSS only yields 16. Can anyone help?

E-mail replies would be much appreciated. I can be reached at larry_sonna@yahoo.com and at larry_sonna@hotmail.com

Thanks in advance,

Larry Sonna

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Sep 10 07:27:26 2005

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