data.txt

1  3.0
0.95  10.0
0.90  15.0
0.85  8.0
0.80  7.
0.75  6.0
0.70  5.0
0.65  4.0
0.50  3.0
0.45  3.0
0.4  3.0

media.c

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include <time.h>
#define VMAX 1000
#define NMAX 67.0
int main()
{
    int j;
    double average,stddev, S[VMAX], Freq[VMAX], Dist[VMAX];
    FILE *statdata, *data;

    data=fopen("data.txt","r");
    statdata=fopen("stat-data.txt","w");

    while(fscanf(data,"%lf  %lf", &S[j], &Freq[j]) > 0) {
      // fscanf(data,"%lf", &Freq[j]);
      Dist[j]=fabs(S[j]*Freq[j]-NMAX);
    }



    for(j=0, average=0.0;NMAX>j;j=j+1)
      average=Dist[j]+average;

    average=average/NMAX;
    for(j=0, stddev=0.0;NMAX>j;j=j+1)
      stddev=(average-Dist[j])*(average-Dist[j])+stddev;

    stddev=stddev/NMAX;
    stddev=sqrt(stddev);

    if(average-(stddev*3)<0)
      fprintf(statdata,"the sample is very bad, the average distance is %g and the standard deviation is %g",average, stddev);
    else if(average-(stddev*3)>1)
      fprintf(statdata,"the sample is very good, the average distance is %g and the standard deviation is %g",average, stddev);
    else
      fprintf(statdata,"the sample is ok, the average distance is %g and the standard deviation is %g. We recommend to add more data",average, stddev);

    fclose(data);
    fclose(statdata);
    //system("PAUSE");
    return 0;
}

the sample is very bad, the average distance is 0.98209 and the standard deviation is 7.97853%