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
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