R help - Jun 2018 - specifying random effects covariance structure in nlme

Hi

I'm trying to fit a mixed effects exponential decay model, in which I 
have random effects for the initial value (init), the asymptote (asymp), 
and the rate (rate).

The catch is that I'd also like to estimate the correlation between init 
and asymp, but not between init and rate, or asymp and rate.

Now using? random = pdDiag(init + asymp + rate ~ 1) has none of the 
random effects correlated

And using? random = pdSymm(init + asymp + rate ~ 1) has all three of the 
random effects correlated

How do I specify just the correlation I want?

cheers

Chris

I haven't played with this for a decade or so, but I believe you can do
something with pdBlocked(). Possibly ask over on R-sig-ME as this quickly gets
beyond the R-help level.

(Also, you are aware that it is not about what you want to estimate, but whether
you believe the correlations are nonzero?)

-pd
> On 16 Jun 2018, at 13:00 , Chris Stride <c.b.stride at
sheffield.ac.uk> wrote:
> 
> Hi
> 
> I'm trying to fit a mixed effects exponential decay model, in which I
have random effects for the initial value (init), the asymptote (asymp), and the
rate (rate).
> 
> The catch is that I'd also like to estimate the correlation between
init and asymp, but not between init and rate, or asymp and rate.
> 
> Now using  random = pdDiag(init + asymp + rate ~ 1) has none of the random
effects correlated
> 
> And using  random = pdSymm(init + asymp + rate ~ 1) has all three of the
random effects correlated
> 
> How do I specify just the correlation I want?
> 
> cheers
> 
> Chris
> 
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