Re: Percentiles

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On Fri, Mar 26, 2004 at 10:57:02AM -0400, Michael Kiefte wrote:

     At least my definition is consistent with SPSS. =20

I'm not convinced !!!


I don't have a copy of SPSS, but I have a tutorial book, which uses
the following example data set:

 75.00 ,
 66.00 ,
 47.00 ,
 80.00 ,
 72.00 ,
 51.00 ,
 43.00 ,
 60.00 ,
 75.00 ,
 70.00 ,
 30.00 ,
 68.00 ,
 80.00 ,
 56.00 ,
 77.00 ,
 64.00 ,
 74.00 ,
 76.00 ,
 45.00=20

I ran this through your algorithm and got the 25th percentile:  53.50

In my book however, the sample SPSS printout gives 51.00 which is what
you'd get if you used the more naive algorithm:


double=20
pctile(double *x, size_t n, double q)
{
  /* make sure q is between 0.0 and 1.0 inclusive */

  double idx =3D (n-1)*q;
  int lo =3D (int) idx;

  qsort(x, n, sizeof(double), cmp);

  return x[lo];
}  =20


The book says it's based on SPSS Version 11.0=20
If anyone's got a copy of SPSS would they care to try this:=20

DATA LIST LIST  /SALES * .
BEGIN DATA.
 75.00=20
 66.00=20
 47.00=20
 80.00=20
 72.00=20
 99.00=20
 51.00=20
 43.00=20
 60.00=20
 75.00=20
 70.00=20
 30.00=20
 68.00=20
 80.00=20
 56.00=20
 77.00=20
 64.00=20
 74.00=20
 76.00=20
 45.00=20
END DATA.

MISSING VALUE sales (99.0) .

FREQUENCIES
        VARIABLES=3Dsales
 	/PERCENTILES =3D 25 50 75.

FINISH.



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