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

And usingĀ  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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