[R] Problems with lm()
Peter Dalgaard
p.dalgaard at biostat.ku.dk
Sat Jun 21 10:00:52 CEST 2008
<oops, this got sent as reply to Andrew only>
Andrew Robinson wrote:
> In your data, subject is nested within sequence. Was that your
> intention?
>
>
Presumably yes. This looks like a standard cross-over design.
I fail to see what the interaction between subject and sequence might
mean, so I also have no idea what SPSS might have done. (The list does
not allow .doc attachments).
Since this is a balanced design, you can get almost the right analysis by
> m <- lm(Max ~ sequence + subject + period +
+ drug, data=Data)
> anova(m)
Analysis of Variance Table
Response: Max
Df Sum Sq Mean Sq F value Pr(>F)
sequence 1 585 585 0.0160 0.9014
subject 12 634325 52860 1.4469 0.2660
period 1 63175 63175 1.7293 0.2131
drug 1 58149 58149 1.5917 0.2311
Residuals 12 438395 36533
in which you have to know that sequence is aliased with the drug:period
interaction, AND that it needs to be compared with the intersubject
variation. I.e. the F test is wrong and should be replaced with F=
585/52860 = 0.011on 1 and 12 df.
However, a better way is
> summary(aov(Max ~ period*drug + Error(subject), data=Data))
Error: subject
Df Sum Sq Mean Sq F value Pr(>F)
period:drug 1 585 585 0.0111 0.918
Residuals 12 634325 52860
Error: Within
Df Sum Sq Mean Sq F value Pr(>F)
period 1 63175 63175 1.7293 0.2131
drug 1 58149 58149 1.5917 0.2311
Residuals 12 438395 36533
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907
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