Dan,
It is hard to say without being able to reproduce your example. If you send me
the data
I could try to advise something.
Roger
url: www.econ.uiuc.edu/~roger Roger Koenker
email rkoen...@uiuc.edu Department of Economics
vox: 217-333-4558 University of Illinois
fax: 217-244-6678 Urbana, IL 61801
On Feb 13, 2012, at 7:50 AM, Dan Morovitz wrote:
> Hi,
>
>
> I'm attempting to calculate the 0.25 and 0.97 quantiles for tree height (0-50
> meters) against tree age (0-300 years) and I am running into some difficulty
> with the plotted grafic. I've run the examples in the quantreg help and can
> get those to work properly and by plugging in my data I can also get the
> lines plotted on my dataset. Unfortunately I'm running into a problem with
> the median and other regression lines with tree age younger than 50 years,
> basically the median is in this range overestimated and even comes out of the
> rage of oberservations. here is the code I'm using.
>
>
> # then fit the median using nlrq
>
> spruce.nlrq <- nlrq(height ~ SSlogis(age, Asym, mid, scal),data=spruce,
> tau=0.5, trace=TRUE)
> lines(1:200, predict(spruce.nlrq, newdata=list(age=1:200)), col="2")
>
>
> I believe this has something to do with the SSlogis, as this gives the
> parameters for an S curve. My data set does not have the typical S curve
> shape, instead you could image it as starting at the inflection point of an S
> curve. This is what I expect the .025 quantile to be similar to:
>
> x <- seq(1,100,1)
> plot(log(x))
>
>
> the 0,975 should also have a logarithmic shape but no such a steep incline.
> Ive tried using different self starting models as found under:
>
> apropos("^ss")
>
>
> however I have not gotten them to work, or at least to fix the problem.
> Curves similar to mine look like these site index curves:
> http://www.extension.umn.edu/distribution/naturalresources/components/3473-13.html
>
> In the example from the nlrq help the lines of the median and the various
> quantiles all start from the same location, tha is basically (x=0, y=0) in
> the coodinate plane. With my problem, the lines start to be drawn from
> various different positions ( the lines always start at age(x)=0, but the
> height(y) can range between 0 and 15).
> Additionally, the data set is quite large. with about 50,000 oberservations
> on age and height.
> Does anyone have a suggestion on how to fix this problem?
> Thanks in advance
> Dan
>
> [[alternative HTML version deleted]]
>
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