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Subject:

simple non-linear model

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Date:

Fri, 2 Jun 2006 09:13:35 +0100

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 ```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] __________________________________________________________ ------------------------------------------------------------------- This list is for discussion of modelling issues and the BUGS software. For help with crashes and error messages, first mail [log in to unmask] To mail the BUGS list, mail to [log in to unmask] Before mailing, please check the archive at www.jiscmail.ac.uk/lists/bugs.html Please do not mail attachments to the list. To leave the BUGS list, send LEAVE BUGS to [log in to unmask] If this fails, mail [log in to unmask], NOT the whole list ```