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

Thierry Onkelinx th|erry@onke||nx @end|ng |rom |nbo@be
Wed Mar 3 13:30:36 CET 2021

Dear Iago,

We only have your description of the data. It would be easier for us to
help you if you provide a small dataset that illustrates the structure in
your data and how you use the data in your model.

Best regards,


ir. Thierry Onkelinx
Statisticus / Statistician

Vlaamse Overheid / Government of Flanders
Team Biometrie & Kwaliteitszorg / Team Biometrics & Quality Assurance
thierry.onkelinx using inbo.be
Havenlaan 88 bus 73, 1000 Brussel

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to say
what the experiment died of. ~ Sir Ronald Aylmer Fisher
The plural of anecdote is not data. ~ Roger Brinner
The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of data.
~ John Tukey


Op wo 3 mrt. 2021 om 13:11 schreef IAGO GINÉ VÁZQUEZ <i.gine using pssjd.org>:

> 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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