# Re: [R] question on graphs and finding area under a curve

Date: Wed 03 Aug 2005 - 20:13:29 EST

Also see function rocdemo.sca in the ROC package.

The area under the 45 degree line in an ROC curve has an area of 0.5.

On Tue, 2005-08-02 at 09:24 -0400, Ravi Varadhan wrote:
> Hi,
>
> To find the area lying between the curve y = y(x) and 45 degree line (which,
> assuming it goes through the origin, is y = x), you can use the following
> function based on trapezoidal rule:
>
> trap.rule <- function(x,f) {sum(diff(x)*(f[-1]+f[-length(f)]))/2}
>
> trap.rule(x,f=y-x)
>
> This area will be negative if y(x) is below the 45 degree line.
>
> However, your question is not complete, I think. You need to specify the
> interval of integration. For this you may need to determine the points of
> intersection of the two curves, which involves the solution of a fixed point
> problem.
>
> Hope this helps,
> Ravi.
>
> > -----Original Message-----
> > From: r-help-bounces@stat.math.ethz.ch [mailto:r-help-
> > bounces@stat.math.ethz.ch] On Behalf Of Renuka Sane
> > Sent: Tuesday, August 02, 2005 8:51 AM
> > To: r-help@stat.math.ethz.ch
> > Subject: [R] question on graphs and finding area under a curve
> >
> > Question on graphs:
> >
> > The default case for drawing a graph in R, is where a little space is left
> > on the x and y axis before the first tick i.e. even if I say xlim=c(0,1) -
> > -
> > there will be some space between the edge of the x-axis and where 0 is
> > placed. If I want 0 on the edge, how do I do it in R?
> >
> > Area under the curve:
> >
> > I have a 45 degree line and a curve above or below it. Is there a way in R
> > to find the area between the two?
> >
> > Thanks,
> > Renuka
> >
> > --
> > Renuka Sane
> > http://www.nyx.net/~rsane
> >
> > [[alternative HTML version deleted]]
> >
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