OpenBUGS seems to require the initial values for intrinsic CAR effects to sum to zero, whereas WinBUGS doesn't. If I need "n" initial values for such random effects, I have had good luck randomly generating n-1 of them and setting the last one to minus the sum of the others.
________________________________________
From: (The BUGS software mailing list) [[log in to unmask]] on behalf of Neil Walker [[log in to unmask]]
Sent: Wednesday, November 23, 2011 11:13 AM
To: [log in to unmask]
Subject: initial values for intrinsic CAR effects
Hello BUGS,
I am currently trialling and refining a model in OpenBUGS in which one of the priors is an intrinsic CAR distribution. If I randomly generate initial values for the CAR effects (I simulated from a Normal with mean 0 and a small variance for all CAR effects), the analysis stalls and I get a message to the effect "initial value for car[1] not feasible". I can get round this by setting all initial values for CAR effects to zero but it would be preferable if I could vary these in the usual way. The other thing to add is that I can successfully run the same model in WinBUGS with randomly generated CAR effects.
Am I missing something very obvious?
Any help appreciated.
regards,
Neil
--
Neil Walker, MSc
Statistician
The Food and Environment Research Agency
Woodchester Park,
Tinkley Lane,
Nympsfield,
Stonehouse,
GL10 3UJ
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