ssxstats.c

/*******************************************************************************

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*******************************************************************************/

/***********************************************************************
      LIBRARY: UTIL - General Purpose Utility Routines

      FILE:    SSXSTATS.C
      AUTHOR:  Darrin Dimmick
      DATE:    04/25/90

      Contains general purpose routines for computing variance
      based statistics using simple numerical methods.

      ROUTINES:
#cat: ssx_stddev - computes standard deviation based on sum of samples
#cat:              and sum of the square of samples
#cat: ssx_variance - comptues variance based on sum of samples
#cat:              and sum of the square of samples
#cat: ssx - fundamental computation to support variance and stddev
#cat:

***********************************************************************/

#include <stdio.h>
#include "util.h"
#include <math.h>

/*****************************************************/
/* Routine: Standard_deviation                       */
/* Author:  Darrin L. Dimmick                        */
/* Date:    4/25/90                                  */
/*****************************************************/
/*****************************************************
   ssx_stddev() accepts the sum of the
   values, the sum of the squares of the values, and
   the number of values contained in the sum and
   returns the standard deviation of the data.

   double sum_x;  # sum of the x values
   double sum_x2; # sum of the squares of the x values
   int    count;  # number of items sumed
******************************************************/
double ssx_stddev(const double sum_x, const double sum_x2,
                          const int count)
{
  double var = 0;

  var = ssx_variance(sum_x,sum_x2,count);
  if (var >= 0.0)
     return (sqrt(var));
  else
     /* otherwise error code */
     return(var);
}

/*****************************************************/
/* Routine: Variance                                 */
/* Author:  Darrin L. Dimmick                        */
/* Date:    4/25/90                                  */
/*****************************************************/
/*****************************************************
   ssx_variance() accepts the sum of the values, the sum
   of the squares of the values, and the number of
   values contained in the sum and returns the
   variance of the data.

   double sum_x;   # sum of the x values
   double sum_x2;  # sum of the squares of the values x
   int    count;   # number of items that were sumed
******************************************************/
double ssx_variance(const double sum_x, const double sum_x2, const int count)
{
  double ssxval;   /* holds value from SSx() */
  double variance; 

  if (count < 2){  
     fprintf(stderr,"ERROR : ssx_variance : invalid count : %d < 2\n", count);
     return(-2.0);
  }
  ssxval = ssx(sum_x,sum_x2,count);
  variance = ssxval/(count-1);

  return(variance);
}

/*****************************************************/
/* Routine: SSx                                      */
/* Author:  Darrin L. Dimmick                        */
/* Date:    4/25/90                                  */
/*****************************************************/
/*****************************************************
   ssx() accepts the sum of the values, sum_x, the
   sum of the squares of the values, sum_x2 and the
   number of values contained in the sums, count and
   returns the value of the sum of the squares
   calculation, SS(x).

   double sum_x;  # sum of x  values
   double sum_x2; # sum of the squares of the x values
   int count;     # number of values sumed
******************************************************/
double ssx(const double sum_x, const double sum_x2, const int count)
{ /* SS(x) SS(y) */
  /* SS(x) = (sum_x2 - ((sum_x * sum_x)/count)) */
  double ssx;

  ssx = sum_x * sum_x;
  ssx = ssx/count;
  ssx = sum_x2 - ssx;

  return (ssx);
}