Re: constraining correlation = 1

Dear Lutz,

re:
I am trying to test the hypothesis that the additive genetic
correlation in my bivariate trait animal model is different from 1.

For the significance of its difference from 0, I can simply specify a
starting value of 0.0 for the covariance term in the variance
structure specification and use a qualifier like !GUFU to keep it
fixed at 0.

But I don't find a way to fix the correlation = 1. Constraining the
two variances and their covariance to be equal (using a VCC line
-depending on the exact model- like 13 14 15) won't converge in any
case. Essentially, this is also asking too much, since for cor12=1, it
is not really necessary to have var1=var2, right?

Are there any options that I'm not aware of, or does my approach look
simply wrong altogether? Any comments would be highly appreciated.

Further to Ari's response

The way to fit a model with correlation of 1 is to use XFA with zero PSI's

Thus, replace the model term Tr.animal with xfa(Tr,1).animal

and define its G structure as

xfa(Tr,1).animal 2
xfa(Tr,1) 0 XFA1
0 0 a b # replace a and b with SQRT of the respective genetic
variances
animal 0 AINV

Qualifications:
1) XFA crossed with relationship matrix does not work in ASReml 1
2) There was a bug in the LogL calculation for XFA with zero PSI crossed
with NRM
which is fixed in ASReml 2.00 build am ; this will affect any LogL tests
you might make.

Grace, mercy and peace to you from God our Saviour Jesus Christ

Arthur Gilmour PhD
mailto:Arthur.Gilmour_at_dpi.nsw.gov.au
Principal Research Scientist (Biometrics)
NSW Department of Primary Industries
Orange Agricultural Institute, Forest Rd, ORANGE, 2800, AUSTRALIA

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Received on Mon Nov 29 2006 - 09:30:41 EST