Xu, Xiuli (NIH/NHLBI) [E] wrote: > I would really appreciate it if someone can give suggestions on how to > do spectra fitting in R using ordinary least square fitting and > non-negativity constraints. The lm() function works well for ordinary > least square fitting, but how to specify non-negativity constraints? It > wouldn't make sense if the fitting coefficients coming out as negative > in absorption spectra deconvolution. > > Thanks. > > Xiuli >
I'm not sure, but would presume that constraints could not be imposed on a linear least squares fit. maybe someone can correct me. if you move to `nls', i.e. non-linear least squares fitting, you should be able to transform your model function. say, you want some parameter `a' to stay positive. then you could e.g. substitute `a = exp(b)' in the model function and fit `b' without constraints in the "new" model and calculate `a' afterwards (which obviously is guaranteed now to be positive). note that error estimates would than have to be computed by gaussian error propagation from `b' to `a'. joerg ______________________________________________ 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
