Hello all! Consider a dataset with a grouping structure, Group (factor) Several treatments, Treat (factor) Some sort of yield, Yield (numeric) Something, possibly important, measured for each group; GroupCov (numeric)
To look for fixed effects from Treat on Yield, a first attempt could be: m1 <- lm(Yield ~ Treat) which gives, in a symmetric situation, the same estimated fixed effects as: m2 <- lme(Yield ~ Treat, random =~1| Group) but m2 is a much better model with safer significances. Now I want to evaluate GroupCov as a covariate to Treat. I can then start with either m1 or m2 as base, but what is most correct when GroupCov has only one value for each Group? m3 <- lm(Yield ~ Treat + GroupCov + Treat:GroupCov) gives the same fixed effects as m4 <- lme(Yield ~ Treat + GroupCov + Treat:GroupCov, random =~1| Group) but this time the prob.values for GroupCov is much stronger in m3 than in m4. Needless to say, anova(m3,m4) tells that m4 is a better *model* than m3. But is it better for my purpose? Trying an old-fashioned style model with only fixed effects? (Don´t shout at me, it is only a dirty test of the system): m5 <- lm(Yield ~ Group + Treat + GroupCov + Treat:GroupCov) is accepted by lm() but here GroupCov is silently removed from the analysis by lm(). I accept this removal, but I get even more suspicious that something fishy is going on in m4. My gut-feeling is that m3 is the right starting point but I have got a general recommendation always to start with m4-type calls when evaluating numeric covariates. I need one (or two) "doctor second opinion" on this. How is the mathematics inside lme() working? Is some part of the variation I want to catch as an effect from GroupCov already removed by the random call, or why do I get better significances in the pure fixed call? Could these sigificances be some sort of artefact? Cheers /CG PS. I sent basically this question, but in a more special case and with another header, to the list two days ago. Nobody was interested, hopefully this is more tasty ;-) DS. -- CG Pettersson, MSci, PhD Stud. Swedish University of Agricultural Sciences (SLU) Dep. of Crop Production Ekology. Box 7043. SE-750 07 Uppsala, Sweden [EMAIL PROTECTED] ______________________________________________ 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