Dear WinBUGGERS,
I am new to WinBUGS and have tried a full day solving this seemingly simple problem and would be very grateful if somebody could point out where my mistake is.
I want to estimate a non-linear relationship between fat mass extracted from 55 garden warblers and fresh body mass. My physiologist colleagues suggested the following function:
y = A + (B + C*x - A)/(1 + exp(D - E*x)),
where x is fresh body mass, y is fat extracted and A, B, C, D, and E are parameters. This curve has the desirable properties of having an asymptote at A (expected to lie between 0 and 0.5) and a constant slope of between 0 and 1 at higher values of x.
MLEs that somebody obtained years ago (A=0.22, B=0.81, C=0.48, D=8.9, E=0.35) seem to produce a well-fitting curve (albeit with heteroscedastic errors). However, I just can't replicate them in WinBUGS, using this model:
model {
A ~ dnorm(0,.01)
B ~ dnorm(0,.01)
C ~ dnorm(0,.01)
D ~ dnorm(0,.01)
E ~ dnorm(0,.01)
tau.fat ~ dgamma(.01,.01)
for(i in 1:55){
fat[i] ~ dnorm(mu[i],tau.fat)
mu[i] <- A + ( B + C*fresh[i] - A )/( 1 + exp(D - E*fresh[i]))
}
}
Apart from not getting similar estimates for A-E, two parallel chains just don't seem to mix for D and E, regardless if I scale the covariate or not.
Thanks a lot for any suggestions about what may be wrong.
Kind regards -- Marc
__________________________________________________________
Marc Kery
Schweizerische Vogelwarte - Swiss Ornithological Institute
CH - 6204 Sempach
Switzerland
phone: 0041 41 462 97 93 (direct)
fax: 0041 41 462 97 10
email (work): [log in to unmask]
email (private): [log in to unmask]
__________________________________________________________
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