[R-sig-ME] specifying independence of random effects?

[ONKELINX, Thierry]

Dear Rob,

I suppose that IMPAIRMENT is a continunous variable. In that case you
can use (IMPAIRMENT - 1|DISCAT) + (1 |DISCAT) to indicate that the
random intercept and the random slope are independent. AFAIK you can
only specify a full variance-covariance matrix between random effects or
independent random effects. So structures like pdIdent, pdCompSymm, ...
are not available in lme4.

HTH,

Thierry


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek
team Biometrie & Kwaliteitszorg
Gaverstraat 4
9500 Geraardsbergen
Belgium

Research Institute for Nature and Forest
team Biometrics & Quality Assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium

tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

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

-----Oorspronkelijk bericht-----
Van: r-sig-mixed-models-bounces at r-project.org
[mailto:r-sig-mixed-models-bounces at r-project.org] Namens Rob Forsyth
Verzonden: donderdag 10 december 2009 17:31
Aan: r-sig-mixed-models at r-project.org
Onderwerp: [R-sig-ME] specifying independence of random effects?

Hello

I'm trying to specify a mixed-effect model describing "unmet NEED" (of
families with disabled children) as a function of IMPAIRMENT (i.e.  
intrinsic severity of the child's condition) and AREA of residence
(factor with ~15 levels). The impairment measure is a unidimensional
variable but it needs "calibrating" for separate types of disability
(autism, cerebral palsy etc) represented by another factor, DISCAT

In lmer syntax I have

NEED ~ IMPAIRMENT | DISCAT + 1 | AREA

but I want to specify independence of random effects. I'm aware of the
pdDiag options in the nlme library but unsure how to specify the model
in lme syntax?

Thank you

Rob

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