[R-sig-ME] How to code mixed model with nested factors in lmer

[Federico Calboli]

On 11 Jun 2010, at 19:54, ZHAO, HUIYAN [AG/1005] wrote:
> I have coding question on mixed model in R. I am using R2.11.0 in
> windows. I have an experiment with 2 fixed effect factors - A and B. The
> levels of B are within the levels of A factor. The model is very similar
> to a split plot design except the nesting relationship between the 2
> fixed effect factors. For example: there are 2 levels for A - GM and ZM.
> There are 7 levels of B in total (G1-G4 and Z1 - Z3) - but G1, G2, G3,
> G4 only appear in GM while Z1, Z2 and Z3 only appear in ZM. The SAS code
> is like the following:

> I tried to code this model in R using lmer(). It turned out that R has
> some issues with nesting levels with fixed effect. I changed and tried a
> few models just for diagnostic purpose. And I found whenever I included
> the nesting fixed effects, I got the following error message: "Problem
> in .C("mixed_EM",: Singularity in backsolve, while calling subroutine
> mixed_EM". I am wondering whether someone can help me set up this mixed
> model in R. 


no idea of the error you're getting, but assuming that:

1) B is a *random* factor
2) the levels of B are uniquely coded G1:G4 and Z1:Z3, i.e. GX is only found in level 1 of A and ZX is only found in level 2 of A

my hunch is the formula should be

lmer(y ~ A + (1|B), data)

I am pretty sure the formula for lme (library nlme)

is 

lme(y ~ A, random  = ~1|B, data)

You have not provided us with the code you're running, so I find it difficult to figure out why you get that error. BTW, if B is a *fixed* factor, why not using 

aov(y ~ A + B%in%A, data)

??

F
 
--
Federico C. F. Calboli
Department of Epidemiology and Biostatistics
Imperial College, St. Mary's Campus
Norfolk Place, London W2 1PG

Tel +44 (0)20 75941602   Fax +44 (0)20 75943193

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