Are there any functions to do error propagation in R? I have done a search with little success. Any pointers to read about this topic are greatly welcomed. My specific problem is: I use a linear model (lm) to predict the biomass of an individual in a population, then I add up the biomass of all individuals to calculate the total mass of the population. I want to calculate the error in the final estimate of the total biomass. Also, I have another case where I use a nls model instead of the lm, but in nls se.fit is currently ignored as stated in the help page. Is there an alternative way to calculate the errors in the predictions from nls? Thanks, Manuel