Doran, Harold wrote:
I was able to run the following in nlme successfully, but the same model
and code (same dataset) failed to run in lme4 and gave me the error
message below. Any thoughts?
lme(math~year, data=egsingle, random=~year|schoolid/childid)
Error in lme(formula = math ~ year, data = egsingle, random =
structure(list( :
Unable to invert singular factor of downdated X'X
Is childid unique? If not, you will need to generate a unique id for the child for the lme4 version of lme. (This change is required because the lme4 package can fit models with crossed or partially crossed grouping factors.)
A simple way of creating the necessary factor (thanks to Martin Maechler for showing me this) is
egsingle$chld <- (egsingle$schoolid : egsingle$childid)[drop = TRUE]
(I haven't checked this. I don't have R on the machine that I use for email connectivity. I can check this later today. Is the egsingle data set the same one you sent me earlier?)
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