Further note, that the standard package 'stats4' has a function mle() which was written exactly for problems like yours {and mle() internally calls optim() !}.
But are we sure we are not solving your homework assignment here? >>>>> "Sundar" == Sundar Dorai-Raj <[EMAIL PROTECTED]> >>>>> on Fri, 25 Nov 2005 07:37:42 -0600 writes: Sundar> Adrienne Gret-Regamey wrote: >> Hello: >> >> I am trying to use optim() to estimate the maximum >> likelihood of a function a*x^b = y. Unfortunately, I >> always get the error, that there is no default value for >> b. >> >> Could you give me an example, on how to correctly >> optimize this function with input data x<-c(1,3,11,14). >> >> Thanks a lot, >> >> Adrienne >>>>> "Sundar" == Sundar Dorai-Raj <[EMAIL PROTECTED]> >>>>> on Fri, 25 Nov 2005 07:37:42 -0600 writes: Sundar> Adrienne Gret-Regamey wrote: >> Hello: >> >> I am trying to use optim() to estimate the maximum likelihood of a >> function a*x^b = y. >> Unfortunately, I always get the error, that there is no default value for b. >> >> Could you give me an example, on how to correctly optimize this function >> with input data x<-c(1,3,11,14). >> >> Thanks a lot, >> >> Adrienne Sundar> Problems such as these will have more meaningful responses if you post Sundar> an example (see the posting guide). What's being parameterized here? a Sundar> or b or both? If just 'a', then fix 'b' at the desired value. If both, Sundar> then give optim a starting value. And what is 'y'? Sundar> fn <- function(par, y, x) { Sundar> a <- par[1] Sundar> b <- par[2] Sundar> sum((y - a * x^b)^2) Sundar> } Sundar> x <- c(1, 3, 11, 14) Sundar> y <- exp(rnorm(length(x))) Sundar> ## y ~ a * x^b Sundar> ## log(y) ~ log(a) + b * log(x) Sundar> v <- coef(lm(log(y) ~ log(x))) Sundar> optim(c(exp(v[1]), v[2]), fn, y = y, x = x) Sundar> Again, I may be way off. Please provide an example if this doesn't cover Sundar> what you need. And define all the variables need to run your script. Sundar> --sundar ______________________________________________ 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