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Subject:

another simple problem

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Tue, 8 Dec 2009 18:52:31 +0100

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 ```Dear friends - I have measurements of weight and total water of pig half brains and want to express water/dry matter. My only estimate of dry matter is from subtracting water (by deuterium) from weight. However, I happen to know that dry matter/weight is close to 0.12 with a sd of 0.04. I wonder how to exploit that knowledge to get a better estimate for dry matter than just subtracting water from total weight. I know a bit about the variance both on weighing (tauwgt) and water measurement by deuterium (tauwt) but now just let them be almost unknown. My simple problem is how to employ the knowledge expressed in (WGT[i]-Water[i])/WGT[i] ~dnorm(0.125,716) which is illegal to BUGS. The normal priors for WGT and Water should somehow be replaced by the relationship above. model { for (i in 1:8){ wgt[i] ~ dnorm(WGT[i],tauwgt) H2O[i] ~ dnorm(Water[i],tauwt) #(WGT[i]-Water[i])/WGT[i] ~dnorm(0.125,716) WGT[i] ~ dnorm(0,0.0001) Water[i] ~ dnorm(0,0.0001) FF[i] <- Water[i]/(WGT[i]-Water[i]) } tauwgt ~ dunif(0,1000) tauwt ~ dunif(0,1000) } list(wgt = c(34.65, 42.28, 36.49, 28.19, 31.35, 40.42, 39.09, 31.92), H2O = c(30.20185, 34.70947, 33.08518, 26.39688, 28.06814, 35.27202, 32.90085, 27.18567)) # mean 0.125 sd 0.03737587 then 1/sd^2 er 716 Best wishes Troels -- Troels Ring Heliosvej 12 9210 Aalborg SŲ 98140582 30480176 job: +4599326629 ------------------------------------------------------------------- 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 ```