"Larsen, Thomas" <[EMAIL PROTECTED]> writes: > I collected eggs laid by Springtails everyday over 28 days after swich to > isotopically enriched diet. The eggs were pooled at day 7, 14, and 28 (+ day > 0 = initial value) and analyzed for isotopes. After the diet switch the > isotopic values of the adults and eggs change towards those of the new diet. > Here are the d13C values (y) of the eggs: > > x y > 0 -22.2 > 0 -22.2 > 0 -22.2 > 0 -22.0 > 7 486.9 > 7 498.6 > 7 489.6 > 14 820.9 > 14 817.4 > 28 895.6 > 28 900.7 > 28 890.6 > 28 885.8 > > The y values represent the mean of the sampling period. > > The dataset is very small but previous experiments have shown that a > exponential asymptotic model can be used for this kind of situations. > > How do I fit a model to these pooled values? The y values can be regarded as > the mean of the given sampling period. > > My first guess is that the x values should be in the middle of the collection > period. I call these x-values xi: > xi=c(rep(0,4),rep(3.5,3),rep(10.5,2),rep(21,4)) > > If I fit them to a nonlinear regression model via least squares (NLS) I get > the parameters: > Value Std. Error t value > a 900.386000 3.7839900 237.9460 > b 916.630000 29.3987000 31.1792 > c 0.230811 0.0102677 22.4792 > > How do I procede from here? I should probably use a maximum likelihood > estimate to estimate the fitted xi? > Any help would be greatly appreciated.
My take is that you should just have your expected y values (which in this case are also the eggs-values, but never mind...) modeled as what they are, namely an integral under the curve between x[i-1] and x[i], or for practical purposes take the sum over days (say) 15 to 28 and divide by 14. -- O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
