[R-sig-ME] No data for 1 interaction combination: problem in R not in Genstat
bbolker at gmail.com
Tue Apr 12 15:47:02 CEST 2011
Roger Humphry <roger.w.humphry at ...> writes:
> I've found that when I do a linear mixed model either with lme or lmer
> with interactions amongst some fixed effects that if there are any
> interaction levels unoccupied with data that the model fails.
> "Error in MEEM(object, conLin, control$niterEM) :
> Singularity in backsolve at level 0, block 1" for lme and
> "Error in mer_finalize(ans) : Downdated X'X is not positive definite,
> 6." in lmer.
> Strangely in Genstat the model does produce output.
> My solutions will be either to use Genstat or to create a single new
> factor variable which has the levels of the interaction that *are*
> represented in the interaction.
> I haven't found anything about this when searching. I guess that this
> may be deliberate (e.g. Genstat uses a fudge considered inappropriate)
> but please could anybody advise me?
> I can provide a simple fictitious example that I invented in which the
> interaction would be of interest but can't be modelled. (I haven't
> posted it here because I'm unsure about the etiquette of posting large
> e-mails containing data onto the discussion list).
I don't know that the fudge is inappropriate, it may just not
have been implemented. For example, lm() in base R is sensible
about setting empty factor levels to NA (if Genstat gives you an
*estimate* for the interaction term with the missing levels,
that would be weird).
One way of posting compact examples is to make them from
synthesized data, for example:
d <- expand.grid(f1=LETTERS[1:3],f2=letters[1:3],rep=1:5)
d2 <- subset(d, !(f1=="A" & f2=="a"))
f1 a b c
A 0 5 5
B 5 5 5
C 5 5 5
d3 <- data.frame(d2,y=runif(nrow(d2)))
lm(formula = y ~ f1 * f2, data = d3)
(Intercept) f1B f1C f2b f2c f1B:f2b
0.60964 -0.32352 -0.15239 0.13997 -0.11326 0.10615
f1C:f2b f1B:f2c f1C:f2c
-0.03831 0.47481 NA
showing in this case that lm() does a sensible thing, setting
one confounded parameter to NA.
If you have long examples you can also try posting the data
somewhere and supplying a URL.
Your solutions sound sensible.
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