# Re: setting variance components to be equal

From: Arthur Gilmour <arthur.gilmour_at_CARGOVALE.COM.AU>
Date: Fri, 6 Feb 2009 10:07:10 +1100

Dear Arild,

The easiest way to do this is

example code:
T1 T2 ~ Fixed !r Trait.animal

1 2 1
0
2 0 US !GPZP
3*0

Trait.animal 2
2 0 US !GPZP !=101
3*0
animal 0 AINV

So, without the !=101
you have separate error variances and genetic variances
and NO covariance.

With the !=101
the genetic variances are the same.

Put !=101 on the R structure to force the error variances to be the
same.

If there are error links (both traits on the same animals)

Of course there may be just scale differences between the traits
but the same heritability (ratio of genetic to error).
It is harder to test that model becasue ASReml does not have a
proportionality constraint. The only way I can think would be to do a
manual grid search using !VCC to define a common ratio between the R
variances and the G variances. !VCC is much harder to use successfully.

>
> at(Trait,1).animal 2
> 1 2 US
> 5
> animal
>
> at(Trait,2).animal 2
> 1 2 US
> 5
> animal

On Thu, 2009-02-05 at 19:26 +0000, ahu wrote:
> I would like to constrain two of my variance components in the model
> to optimise LogL assuming they are equal (doesn't matter what value
> they take). How does one code this in ASreml?
>
> Example: Two populations and a bivariate model of the same trait.
> Question: do the two pop differ signf from each other in Va?
>
> example code:
> T1 T2 ~ Fixed !r at(Trait,1).animal at(Trait,2).animal
>
> 1 1 2
>
> at(Trait,1).animal 2
> 1 2 US
> 5
> animal
>
> at(Trait,2).animal 2
> 1 2 US
> 5
> animal
>
> How could I specify that the two populations should have equal Va
> without actually giving them a number? I.e is it possible to say that
> the two var comp should be the same, get the LogL and compare to a
> unconstrained model. If no better fit then that suggests that Va are
> not signf different in the two pop.
>
> Thanks,
> Arild

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Received on Sat Feb 06 2009 - 10:07:10 EST

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