On Fri, Apr 15, 2011 at 8:45 AM, Ben Bolker <bbol...@gmail.com> wrote: > Kevin Wright <kw.stat <at> gmail.com> writes: > >> >> > I am trying to teach myself R and replicate some previous SAS analysis. >> > Could someone please help me translate the following SAS code into R. >> > >> > Proc mixed method=ml >> > Class Group Treatment Stream Time Year; >> > Model Logrpk=Treatment Time Treatment*Time; >> > Random Group Stream (Group Treatment) Year(Time); >> > >> >> Assuming you have a data frame "dat" with these factors: Group Treatment >> Stream Time Year >> And continuous response: logrpk >> >> This code is a starting point: (I'm not sure exactly what the SAS syntax >> means). >> >> require(lme4) >> m1 = lmer(logrpk ~ treatment*time + (1|Group) + (1|Stream:Group:Treatment) + >> (1|Year:Time), data=dat) >> > > Can I please suggest that (Treatment|Stream:Group) or something > like it is more appropriate than (1|Stream:Group:Treatment)? In > general, what goes on the LEFT of the bar is an intercept or fixed > effect (i.e. something that varies between groups); what goes on > the RIGHT of the bar is a grouping variable. Thus if a fixed effect > terms ends up on the right of the bar, something funny is going on.
I think it is appropriate to have a fixed-effects term on the right hand side in the form of an interaction. I regard both (Treatment|Stream:Group) and (1|Stream:Group:Treatment) as interactions between a fixed-effects factor (Treatment) and a random-effects factor (Stream:Group). The basic rule is that the interaction between a fixed-effects term and a random-effects term is a random effect. It is not appropriate, however, to have Treatment on the right hand side when it is *not* in an interactions. A formula of Response ~ Treatment + (1|Treatment) + ... is nonsensical. Basically the model with (1|Stream) + (1|Stream:Group) + (1|Stream:Group:Treatment) is a restricted form of the model with (1|Stream) + (Treatment|Stream:Group) in which the variance-covariance matrix for random-effects from the last term has the "compound symmetry" form. It is easier to see this if you write the second term as (0+Treatment|Stream:Group). ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.