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POOdd Pearson correlation coefficient results
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copied!<p>I have an assignment in my C programming class to write a program to get the correlation coefficient of 2 sets of real numbers. I've been given the equations, and it referenced wikipedia so I double checked the equations there. Here is a link to the equation, which seems to be pretty standard from my research:</p> <p><img src="https://upload.wikimedia.org/math/0/4/e/04e3ee493ddb1f01e03d8bf024fbd0a5.png" alt="alt text"></p> <p>I've written the program, but when I ran it I was getting numbers greater than 1 for my results, which I knew wasn't correct. I looked over my code several times but couldn't find anything out of place, so I tried dividing by n at the end instead of n-1, this gave me values with the -1 to 1 range that I expected, so i tested it against data values that I found online as well as a correlation coefficient calculator ( <a href="http://easycalculation.com/statistics/correlation.php" rel="nofollow noreferrer">http://easycalculation.com/statistics/correlation.php</a> ) and I'm now getting correct results for all of the numbers I input. I can't figure out why this is, so thought I might be able to get a little help with it here. Here is my code for the program, If there is anything else that stands out that I have done wrong here I would love to hear some advice, but mostly I'm trying to figure out why I'm getting the right results with what appears to be the wrong equation.</p> <p>It will then read in the values for both arrays(x and y), and then computes the correlation coefficient between the 2 sets of numbers.</p> <pre><code>#include <stdio.h> #include <stdlib.h> #include <math.h> int main(void) { int n; /* value to determine array length */ /* declare variables to hold results for each equation for x and y initialize all to zero to prepare for summation */ float r = 0.0, xbar = 0.0, ybar = 0.0, sx = 0.0, sy = 0.0; /*get number n input from user */ printf("Please enter a number n: "); scanf("%d", &n); if( n < 1) { printf("n must be a positive number.\nPlease enter a new value: "); scanf("%d", &n); if( n < 1) { printf("Invalid input, exiting...\n"); return 0; } } /*initialize arrays x and y with length of n */ float x[n], y[n]; /*use for loop to read in values of x*/ int i; for(i = 0; i < n; ++i) { printf("Please enter a number for x: "); scanf("%f", &x[i]); } /*use for loop to read in values of y*/ for(i = 0; i < n; ++i) { printf("Please enter a number for y: "); scanf("%f", &y[i]); } /*compute xbar */ for(i = 0; i < n; ++i) { xbar += x[i]; } xbar /= n; /*compute ybar*/ for(i = 0; i < n; ++i) { ybar += y[i]; } ybar /= n; /* compute standard deviation of x*/ for(i = 0; i < n; ++i) { sx += (x[i] - xbar) * (x[i] - xbar); } sx = sqrt((sx / n)); /* compute standard deviation of y */ for(i = 0; i < n; ++i) { sy += (y[i] - ybar) * (y[i] - ybar); } sy = sqrt((sy / n)); /*compute r, the correlation coefficient between the two arrays */ for( i = 0; i < n; ++i ) { r += (((x[i] - xbar)/sx) * ((y[i] - ybar)/sy)); } r /= (n); /* originally divided by n-1, but gave incorrect results dividing by n instead produces the desired output */ /* print results */ printf("The correlation coefficient of the entered lists is: %6.4f\n", r); return 0; } </code></pre> <p>(it looks like my code formatting isn't working, very sorry about this. Tried using tags and the button but can't figure it out. It looks like I got it working somewhat, better than before.)</p>

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