[R-sig-ME] fixed-effect model matrix is rank deficient so dropping 1 column / coefficient

[IAGO GINÉ VÁZQUEZ]

Dear all,

I have 3 related questions, probably already answered, but which I cannot find:

When computing a model with lmer I get the message

fixed-effect model matrix is rank deficient so dropping 1 column / coefficient

Then, my questions are, first, how can I see/compute/get the rank deficient fixed-effect model matrix, second how is that matrix computed, and third (these actually are 2 questions), if my model is yet valid (is it?) how can the dropped fixed effect explained in the results of a paper.

In the example in ?fixef
fm2 <- lmer(Reaction ~ Days + Days2 + (1|Subject),
            data=transform(sleepstudy,Days2=Days))
fixef(fm2,add.dropped=TRUE)

the problem happens because 2 independent variables are equal, but in my model the numeric independent variables are not so highly correlated. In fact the problem happens with the interaction between a factor and a numeric variable, since it is one of the categories of the factor interacting with the numeric, which is dropped.

Thank you and stay safe!


Iago


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