On Jun 30, 2010, at 4:02 PM, Asha Sharma wrote:
Hi,
Thanks to both of you for taking the time to answer my question. I
was maybe not very clear in the way I framed my question. By plot, I
meant an x-y plot with a cloud of points which should have a linear
lower envelope. Is there a way to both plot as well as get the
parameters of the lower envelope (intercept, slope, etc.) and to
also set the percentage of points outside it?
I suspect you are requesting a scatterplot with a superimposed linear
fit and 5% and 95% prediction intervals. Not much point in pursuing
until you "confirm or deny".
--
David
Thanks!
Asha
Stephan Kolassa wrote:
Hi,
one possibility would be to calculate the convex hull using
chull(). I believe that the hull points are returned by chull() in
a clockwise order (?), so the points between the rightmost and the
leftmost point in the chull() result are the lower half of the
convex hull. Remove these points from the original dataset (a
variant of "peeling convex hulls") and iterate until you have
removed your prespecified percentage of points - these all lie
outside of the final lower hull (though the percentage will, of
course, only be approximated, but you should be able to modify this
to taste).
HTH
Stephan
David Winsemius schrieb:
On Jun 30, 2010, at 2:05 PM, Asha Sharma wrote:
Hi,
I am looking for a way to find the lower envelope of points on a
plot, preferably specifying what percentage of points should be
allowed to lie outside the envelope. There must be a
straightforward way to do this, but I do not seem to be able to
find it. I would greatly appreciate any help.
You probably want something like the "lower half" of the convex
hull. You should find quite a bit of code with your favorite r
search engine on the topic of "convex hull".
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West Hartford, CT
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