How is the covariance factor computed?

Hi Christian,

Although Vince pretty much answered your question, the preprint that you
refer to provides another description that some might find helpful,

http://arxiv.org/pdf/1406.5823v1.pdf

See "Constructing the relative covariance factor" starting at the bottom of
p. 10 and "PLS step I: update relative covariance factor" at the bottom of
p. 19.

The (profile) likelihood for theta that Vince discussed is equation 34 on
page 16.  Equation 30 is the likelihood for all three types of parameters --
theta, beta, and sigma.

Small point:  I think there is a typo in the answer below in that,

the sigma parameters are then  simply numerically optimized

should refer to theta, not sigma.

Cheers,
Steve


Quoting Vincent Dorie <vdorie at cs.stanford.edu>:

I'm not sure if this answers your question, but the parameters of  the
variance/covariance matrix are stored in the theta slot of a  merMod in a
form such that they correspond to a Cholesky  factorization of the
individual components of the var/cov matrix of  the random effects, with
the diagonal in the first 'd' parts of the  vector and the off-diagonal
stored in column-major format in the  next d * (d - 1) / 2 elements. Given
a theta vector, to get to a  representation such as that which Tlist gives
requires knowing how  to map the parameters to matrices. This is currently
done by hand,  using the knowledge that the cnms slot of a merMod contains
the  dimension of each grouping "factor"/"level" and the aforementioned
Cholesky decomposition storage concept. In the future, however, if  lme4
supports different forms of variance/covariances matrices for  factors
other than "full" (e.g. independent, or correlation only),  then that
knowledge will need to be referenced instead. I believe the!
re is effort on that front in the "flexLambda" branch on github.

On the other hand, if you were asking where those numbers come from,  it
turns out that (at least for linear models) those parameters are
sufficient to define a likelihood wherein the fixed effects and
conditional error term (sigma) are analytically optimized. Since the  goal
is a maximum likelihood, or REML, the sigma parameters are then  simply
numerically optimized. You can then easily evaluate the mixed  model
likelihood at any value of the var/cov matrix of the random  effects that
you like, provided you are willing to accept maximal  values for the fixed
effects and sigma. If you wanted to plug those  values in as well, it's a
bit of a pain but it can be done.

Vince

On Jul 17, 2014, at 11:41 AM, Christian Brauner
<christianvanbrauner at gmail.com> wrote:

Hello,

I am performing a priori power simulations for mixed-effect models based
on previous experiments. This works out quite nicely. I extract parts of
my parameters from a previous model I fitted:

prev_mod <- lmer(Y ~ A
                 + (B | Context)
                 + (B | Subjects),
                 data =3D data)

"A" :=3D 2 level factor
"Context" :=3D 40 level factor
"Subjects" :=3D 70 level factor

create design matrices for the fixed- and each random effect use
functions from
the apply-family and bind them to a previously set-up data frame and so on.

In order to simulate data I draw from two multivariate distributions. One
for Subjects and one for Context. I previously constructed the
variance-covariance matrix by using the estimations I get by issuing
"as.data.frame(prev_mod)". After having read that Doug implemented a new
method for the "getME()" extractor "Tlist" that gives the covariance
factors from which the block matrices in "Lambda" are created I figured I
could get the variance-covariance matrix way easier by doing (code here
only for the "Context" variance-covariance matrix):

cov_fac <- getME(prev_mod, "Tlist")
cov_fac_context <- cov_fac$Context

sigma(prev_mod)^2*cov_fac_context%*%t(cov_fac_context)

But the question that has been haunting me for weeks now is how the
individual
covariance factors (better: "matrices" in this case) that I can extract via
"getME(prev_mod, "Tlist") are computed. Is there some literature on that?  I
couldn't find any apart from the paper "Fitting linear mixed-effects models
using lme4" published 23.06.2014. I would be interested in reading up/get an
explanation how the covariance factor can be computed (mathematically and in
lme4) and if I need the variance-covariance matrix beforehand or vica versa.

Thank for any help!

Best,
Christian Brauner
Eberhard Karls Universit=C3=A4t T=C3=BCbingen
Mathematisch-Naturwissenschaftliche Fakult=C3=A4t - Faculty of Science
Evolutionary Cognition - Cognitive Science
Schleichstra=C3=9Fe 4 =C2=B7 72076 T=C3=BCbingen =C2=B7 Germany
Telefon +49 7071 29-75643 =C2=B7 Telefax +49 7071 29-4721
christianvanbrauner at gmail.com