Re: Percentiles

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On Thu, Mar 25, 2004 at 10:14:42AM -0400, Michael Kiefte wrote:

     > Then I think the way %iles are done in FREQUENCIES is wrong.  Have y=
ou
     > a general definition of the xth percentile ??
    =20
     How's this:
    =20
     double pctile(double *x, size_t n, double q)
     {
     	/* make sure q is between 0.0 and 1.0 inclusive */
    =20
     	double idx =3D (n-1)*q;
     	int lo =3D (int) idx;
     	sort(x, n);
    =20
     	if (idx =3D=3D lo) /* a q of 1.0 will be caught here */
     		return x[lo];
     	/* else */
     	return x[lo] + (x[lo+1]-x[lo]) * (idx-lo);
     }
    =20
By this algorithm, the 0'th percentile always equals the smallest
datum.  Is that what you'd expect.

In my research these last couple of days, I've only found one
statement about percentiles with which I wholly concur:=20

 "There is no universally accepted definition of a percentile"

This comes from http://cnx.rice.edu/content/m10805/latest/ which has
some interesting discussions.

Anyway, I'm running with Michael's definition and I'll have something
checked in soon.

J'



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