On 29/05/11 05:45, Jessica Minkue wrote:
Hello everyone
I really need some help here. I made a confidence ellipse using the function
ellipse from the package ellipse:
ellipse(SD, centre=colMeans(pcsref),t=sqrt((p * (n-1)/(n-p))*qf(0.99, p,n-p))
Now, I want to write a function whom return TRUE or FALSE if a given
observation is in the confidence ellipse. But I have no clue how to do it
Can anyone help me?
First of all, you are probably way off base talking about
***confidence*** ellipses here.
If you are testing whether observations are inside the ellipses, then
you are most
likely interested in ***prediction*** ellipses.
It is vital that you understand the difference.
But to answer your question: It would be easy enough to do it from scratch.
Let your ellipse be defined by
(x - mu)' M(x-mu) = c
where mu is the ``centre'', x is a 2-vector (x_1,x_2)', M is a positive
definite matrix,
c is a positive constant, and " ' " denotes ``transpose''. Then a point
x = (x_1,x_2)' is
inside the ellipse if and only if (x - mu)' M(x-mu) <= c.
Coding this up is an easy exercise. If you can't do it, you probably
shouldn't be messing
with this stuff in the first place.
However if you want to use a sledge-hammer to crack a peanut,
install the "spatstat" package and then do:
require(spatstat)
W <- owin(poly=<your ellipse>)
inside.owin(x1,x2,W) # Where x1 and x2 are the x and y coordinates of
the points you wish to test.
cheers,
Rolf Turner
______________________________________________
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.
