An error in fitting a non linear regression
Hi, Thank you for the reply and suggestions. I have two questions? 1) If I want to use log, it seems that I have to take log from both sides of the model which will lead to lm(log(q)~log(-depth)). What is tehdifference between this syntax and lm(log(q) ~ I(-depth))? 2) How can I calculate the R-squared of a fitted non linear model? Regards Saeed
Christian Ritz-3 wrote:
Hi Saeed, one approach is to try out several initial value combinations for a and b. It often helps to find initial values of the same order of magnitude and of the same sign as the final estimates. To get such initial values, you could linearize the model: lm(log(q) ~ I(-depth)) and supply the estimated coefficients from the linear regression as starting values: nreg <- nls(q ~ a*exp(-b*depth), start = list(a = 0.76168, b = -0.08484)) summary(nreg) Christian
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