[R] Finding Lower Envelope of Points on a Plot?

David Winsemius dwinsemius at comcast.net
Wed Jun 30 22:58:45 CEST 2010

```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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David Winsemius, MD
West Hartford, CT

```

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