[R] intersection of 2 density curves

David Winsemius dwinsemius at comcast.net
Thu Oct 8 17:16:22 CEST 2009


It should also be remembered that there will often be multiple  
intersections of such density estimates.

I hope this is not being done in support of a data-driven  
discretization of two group comparisons. Such practices are to be  
eschewed (as they are even worse than obfuscation).

-- 
David Winsemius.


On Oct 8, 2009, at 10:01 AM, Rainer M Krug wrote:

> On Thu, Oct 8, 2009 at 3:48 PM, Monica Pisica  
> <pisicandru at hotmail.com>wrote:
>
>>
>>
>> Hi,
>>
>> I would like to find out the coordinates of the intersection points  
>> of 2
>> density curves. I did a search but i didn't get any significant  
>> results. I
>> really hope some of you have some ideas. here it is an example:
>>
>> set.seed(123)
>> x1 <- rnorm(100, 1, 1)
>> x2 <- rnorm(100, 0, 1)
>> d1 <- density(x1)
>> d2 <- density(x2)
>>
>> plot(d1, col = 2)
>> lines(d2, col = 4)
>>
>>
> if you specify n, from ant to in density() and keep them the same,  
> then the
> density estimates are done for the same x-coordinates. Therefore you  
> can
> compare d1$y with d2$y to deternine, where they intersect:
>
> d2 <- density(x2, from=-3, to=5, n=2048)
> d1 <- density(x1, from=-3, to=5, n=2048)
> plot(d1, col = 2)
> lines(d2, col = 4)
> d1$y < d2$y
> intersX <- d1$x[as.logical(abs(diff(d1$y < d2$y)))]
> intersY <- d1$y[as.logical(abs(diff(d1$y < d2$y)))]
> abline(v=inters)
> abline(h=intersY)
>
> Cheers,
>
> Rainer
>
>
>> Now i would like to know the coordinates of the intersection point.  
>> I did
>> some "trial and error" and i come up with these .... (that means i  
>> did
>> several vertical and horizontal lines through the graph until  
>> visually i
>> "hit" the intersection point)
>>
>> abline(v = 0.35, lty = 2)
>> abline(h = 0.34, lty = 2)
>>
>> I really appreciate any hints you may have, thanks so much,
>>
>> Monica
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>> ______________________________________________
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>>
>
>
>
> -- 
> Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation  
> Biology,
> UCT), Dipl. Phys. (Germany)
>
> Centre of Excellence for Invasion Biology
> Natural Sciences Building
> Office Suite 2039
> Stellenbosch University
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>
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>
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>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at 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.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT



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