[R] Discrepancy between R and SPSS in 2-way, repeated measures ANOVA
Larry A Sonna
larry_sonna at hotmail.com
Fri Sep 9 16:10:06 CEST 2005
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 3.645
0.0355
TIME 1 473 473 0.175
0.681
COND*TIME 3 975 325 0.120
0.947
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 at yahoo.com and at larry_sonna at hotmail.com
Thanks in advance,
Larry Sonna
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