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Dear BUGS users,

I?m confused about specifying the initial terms in a directed second order 
random walk.  The specification below follows the Ice example in the BUGS 
0.5 Examples Vol 2 (ver. ii):

prec1<-0.000001*tau
b[1]~dnorm(0,prec1)
b[2]~dnorm(0,prec1)
for (i in 3:N) {
        mean[i]<-2*b[i-1]-b[i-2]
        b[i]~dnorm(mean[i],tau)
}
tau~dgamma(0.01,0.01)

The tau in prec1 is explained as being necessary to provide the 
appropriate likelihood for tau.

But I?ve also seen examples where the initial values were specified as 
just:
b[1]~dnorm(0,0.001)
b[2]~dnorm(0,0.001)
omitting the tau term entirely.

And this suggests yet a third specification:
b[1]~dflat()
b[2]~dflat()

I like this third specification because it maintains compatibility with 
including spatial effects via car.normal, which states a dflat intercept 
MUST be specified along with car.normal. 
I?ve tried this and it seems to ?work?, but is it appropriate?

Thanks in advance for any comments. 

Dennis


Dennis Heisey
Research Biologist
USGS National Wildlife Health Center


 


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