Dear list,
I have run two ancova models with the same data, one where the covariate Length
was centered by Site (group mean centered), the other with Length not centered.
The package "car" was used. The models are:
ancova1a <- lm(log10(Weight) ~ LenCtd2 + Site, data = Data2); Length centered
model, Length log10 transformed.
ancova2a <- lm(log10(Weight) ~ log10(Length) + Site, data = Data2); Length not
centered model.
For each model, adjusted mean Weights for each Site were estimated with the
"effects" package. I then extracted the fitted values for each model using
"fitted()" and saved them to a file. For the model ancova1a, the mean of the
fitted Weight values for each Site equalled the adjusted mean Weight for that
Site. For model ancova2a, the fitted values were exactly the same as for model
ancova1a and the Site means thus did not match the comparable adjusted mean
Weights, which differed between models. This puzzles me, and has for some time
as I searched for enlightenment, and I hope that someone can provide an answer
to me. My original intent was to correlate the fitted values from each model
with values of another variable.
Some might recognize the above models as essentially estimating weight at
length or condition in animals, in this case fish.
Thanks for any assistance.
Regards,
B. Jessop
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