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#include <stdio.h> /* Header for input/output subroutines. */
#include <math.h> /* Header for math subroutines. */
#include <float.h> /* Header for floating point subroutines. */
#define pi 3.141592653589793238462643383279 /* Accurate value for pi. */
/* Simpson's rule for approximating integrals.
a: left endpoint
b: right endpoint
fc: pointer to function to integrate
n: number of subintervals
*/
double fc (double x);
main()
double a,b,h,sum,x,y,p,r,psum; /* In 'C' all variables must be assigned */
double p1, p2, p3;
int i, n, o;
printf("\007"); /* Sound bell in computer. */
a = (double) 0.0; /* The normalizing integral is */
b = (double) 1.0; /* from 0 to 1. */
n = 256; /* This has been tested and seen to give 5 decimal accuracy. */
printf("\n Enter number of points ");
scanf("%d",&o);
if (o < 5){
printf("Too few points"); return(o);
p = (double) o;
h = (double) (b-a)/n;
for (i=1, sum=0.0; i<=n; i = i+ 2)
p1 = fc((double) a+(i-1)*h, p);
p2 = fc((double) a+i*h, p);
p3 = fc((double) a+(i+1)*h, p);
sum += p1 + 4.0 * p2 + p3;
psum = (double) h*sum/3.0; /* This normalizes the integral */
/* For a given value of N, P\_N[|r|>=0] (must)) = 1.0 */
/* We assure this (and eliminate the need to compute some */
/* Gamma function) in computing the value in "psum." */
printf("\n Enter correlation coefficient r = ");
scanf("%lf", &r); a = fabs((double) r);
if (a > 1) a = (double) 1.0;
/* r\_0 must be between -1 and 1. Force a to assume a value */
/* between 0 and 1. */
h = (double) (b-a)/n;
for (i=1, sum=0.0; i<=n; i = i+ 2)
p1 = fc((double) a+(i-1)*h, p);
p2 = fc((double) a+i*h, p);
p3 = fc((double) a+(i+1)*h, p);
sum += p1 + 4.0 * p2 + p3;
y = (double) h*sum/3.0;
printf("\n Percentage probability that N measurements");
printf("\n of two uncorrelated variables would give an r");
printf("\n as large as r\_0, %.2lf, that is ", a);
printf("\n P\_N[|r|>=r\_0] = %.2lf (per cent).", (double) 100.*y/psum);
printf("\n If the number %.2lf is less than 5 (per cent),", 100.*y/psum);
printf("\n then the correlation is called significant.");
printf("\n If the number %.2lf is less than 1 (per cent),", 100.*y/psum);
printf("\n then the correlation is call highly significant.");
printf("\n Given this simple program and its speedy calculation");
printf("\n of the significance of a correlation in absolute terms,");
printf("\n we only have to plug in the number of data points and,");
printf("\n the value gotten from Pearson's correlation coefficient, r.");
printf("\n");
return(0);
double fc (double x, double p)
double y;
y = (double) pow((double)(1.0-x*x),(double) (p-4.0)/2.0);
return (y);
/* End of file */
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