[R] nls fitting problems (singularity)

[Gabor Grothendieck]

Have a look at optim (which supports a number of different algorithms via
the method= arg) and segmented in package segmented which does segmented
regression.

For example,

ss <- function(par) {
	b <- par[1]; c1 <- par[2]; c2 <- par[3]; d <- par[4]
	x <- df1$x; y <- df1$y
	sum((y - (d+(x-b)*c1*(x-b<0)+(x-b)*c2*(x-b>=0)))^2)
}
optim(sl,ss)



Karl Knoblick <karlknoblich <at> yahoo.de> writes:

: 
: Hallo!
: 
: I have a problem with fitting data with nls. The first
: example with y1 (data frame df1) shows an error, the
: second works fine.
: 
: Is there a possibility to get a fit (e.g. JMP can fit
: also data I can not manage to fit with R). Sometimes I
: also got an error singularity with starting
: parameters.
: 
: # x-values
: x<-c(-1,5,8,11,13,15,16,17,18,19,21,22)
: # y1-values (first data set)
: y1=c(-55,-22,-13,-11,-9.7,-1.4,-0.22,5.3,8.5,10,14,20)
: # y2-values (second data set)
: y2=c(-92,-42,-15,1.3,2.7,8.7,9.7,13,11,19,18,22)
: 
: # data frames
: df1<-data.frame(x=x, y=y1)
: df2<-data.frame(x=x, y=y2)
: 
: # start list for parameters
: sl<-list( d=0, b=10, c1=90, c2=20) 
: 
: # y1-Analysis - Result: Error in ...  singular
: gradient
: nls(y~d+(x-b)*c1*(x-b<0)+(x-b)*c2*(x-b>=0), data=df1,
: start=sl)
: # y2-Analysis - Result: working...
: nls(y~d+(x-b)*c1*(x-b<0)+(x-b)*c2*(x-b>=0), data=df2,
: start=sl)
: 
: # plots to look at data
: par(mfrow=c(1,2))
: plot(df1$x,df1$y)
: plot(df2$x,df2$y)
: 
: Perhaps there is another fitting routine? Can anybody
: help?
: 
: Best wishes,
: Karl
: 
: 
: 	
: 
: 	
: 		
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