Hi R experts,
I'm trying to use nls() for a piecewise linear
regression with the first slope constrained to 0. There are 10 data
points and when it does converge the second slope is almost always over
estimated for some reason. I have many sets of these 10-point datasets
that I need to do. The following segment of code is an example, and
sorry for the overly precise numbers, they are just copied from real
data.
y1<-c(2.37700445, 1.76209775, 0.09795576, 2.21834963, 6.62262243,
15.70471269, 21.92956392, 36.39401717, 32.43620195, 44.77442277)
x1<-c(24.6,
28.9, 33.2, 37.6, 42.0, 46.4, 50.9, 55.3, 59.8, 64.3)
dat <-
data.frame(x1,y1)
nlmod <- nls(y1 ~ ifelse(x1 < xint+(yint/slp), yint, yint +
(x1-(xint+(yint/slp)))*slp),
data=dat,
control=list(minFactor=1e-5,maxiter=500,warnOnly=T),
start=list(xint=39.27464924, yint=0.09795576, slp=2.15061064),
na.action=na.omit, trace=T)
##plotting the function
plot(dat$x1,dat$y1)
segments(x0=0,
x1=coef(nlmod)[1]+coef(nlmod)[2]*coef(nlmod)[3],
y0=coef(nlmod)[2], y1=coef(nlmod)[2])
segments(x0=coef(nlmod)[1]+coef(nlmod)[2]*coef(nlmod)[3],x1=80,
y0=coef(nlmod)[2], y1=80*coef(nlmod)[3]+coef(nlmod)[2])
As
you can see from the plot, the line is above all data points on the
second segment. This seems to be the case for different datasets. I'm
wondering if anyone can help me understand why this happens. Is this
because there are too few data points or is it because the likelihood
function is just not smooth enough?
Karen
_________________________________________________________________
The New Busy is not the old busy. Search, chat and e-mail from your inbox.
N:WL:en-US:WM_HMP:042010_3
[[alternative HTML version deleted]]
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
