Computing Correlations Within Respondents

The default methods in Q for computing Correlations are designed to compute the correlations between variables. It is possible to compute the correlations between sets of data within respondents using JavaScript Variables.

Pearson's correlation

The following script computes Pearson's Correlation between two sets of variables for each respondent.

var v1 = [Q5_5_1,Q5_5_2,Q5_5_3,Q5_5_4,Q5_5_5,Q5_5_6];
var v2 = [q2a,q2b,q2c,q2d,q2e,q2f];
pearsonCorrelation = function(x, y) 
{// source http://stevegardner.net/2012/06/11/javascript-code-to-calculate-the-pearson-correlation-coefficient/
	var shortestArrayLength = 0;
	if(x.length == y.length)
		shortestArrayLength = x.length;
	else if(x.length > y.length)	{
		shortestArrayLength = y.length;
		console.error('x has more items in it, the last ' + (x.length - shortestArrayLength) + ' item(s) will be ignored');}
	else{
		shortestArrayLength = x.length;
		console.error('y has more items in it, the last ' + (y.length - shortestArrayLength) + ' item(s) will be ignored');}
	var xy = [];
	var x2 = [];
	var y2 = [];
	for(var i=0; i<shortestArrayLength; i++)	{
		xy.push(x[i] * y[i]);
		x2.push(x[i] * x[i]);
		y2.push(y[i] * y[i]);}
	var sum_x = 0;
	var sum_y = 0;
	var sum_xy = 0;
	var sum_x2 = 0;
	var sum_y2 = 0;
	for(var i=0; i<shortestArrayLength; i++)	{
		sum_x += x[i];
		sum_y += y[i];
		sum_xy += xy[i];
		sum_x2 += x2[i];
		sum_y2 += y2[i];
	}
	var step1 = (shortestArrayLength * sum_xy) - (sum_x * sum_y);
	var step2 = (shortestArrayLength * sum_x2) - (sum_x * sum_x);
	var step3 = (shortestArrayLength * sum_y2) - (sum_y * sum_y);
	var step4 = Math.sqrt(step2 * step3);
	var answer = step1 / step4;
	return answer;}

pearsonCorrelation(v1, v2)

Spearman's correlation

Please note that the following formula will be incorrect where there are ties in the data (and, it is incorrect in the example below for this reason).

var v1 = [Q5_5_1,Q5_5_2,Q5_5_3,Q5_5_4,Q5_5_5,Q5_5_6];
var v2 = [q2a,q2b,q2c,q2d,q2e,q2f];
ranking = function(arr) {//adapted from http://stackoverflow.com/questions/14834571/ranking-array-elements
   var sorted = arr.slice().sort(function(a,b){return b-a})
   return arr.slice().map(function(v){ return sorted.indexOf(v)+1 });
}
spearmanCorrelation = function(input_x, input_y) 
{// adapted from http://stevegardner.net/2012/06/11/javascript-code-to-calculate-the-pearson-correlation-coefficient/
	x = ranking(input_x);
	y = ranking(input_y);    
    var shortestArrayLength = 0;
	if(x.length == y.length)
	{
		shortestArrayLength = x.length;
	}
	else if(x.length > y.length)
	{
		shortestArrayLength = y.length;
		console.error('x has more items in it, the last ' + (x.length - shortestArrayLength) + ' item(s) will be ignored');
	}
	else
	{
		shortestArrayLength = x.length;
		console.error('y has more items in it, the last ' + (y.length - shortestArrayLength) + ' item(s) will be ignored');
	}
 
	var xy = [];
	var x2 = [];
	var y2 = [];
 
	for(var i=0; i<shortestArrayLength; i++)
	{
		xy.push(x[i] * y[i]);
		x2.push(x[i] * x[i]);
		y2.push(y[i] * y[i]);
	}
 
	var sum_x = 0;
	var sum_y = 0;
	var sum_xy = 0;
	var sum_x2 = 0;
	var sum_y2 = 0;
 
	for(var i=0; i<shortestArrayLength; i++)
	{
		sum_x += x[i];
		sum_y += y[i];
		sum_xy += xy[i];
		sum_x2 += x2[i];
		sum_y2 += y2[i];
	}
 
	var step1 = (shortestArrayLength * sum_xy) - (sum_x * sum_y);
	var step2 = (shortestArrayLength * sum_x2) - (sum_x * sum_x);
	var step3 = (shortestArrayLength * sum_y2) - (sum_y * sum_y);
	var step4 = Math.sqrt(step2 * step3);
	var answer = step1 / step4;
 
	return answer;
}

spearmanCorrelation(v1, v2)

How to use these scripts

1. Insert a new JavaScript Variable.
2. Enter this the script below into the Expression box.
3. Replace the Variable Names in the first two rows with the ones you wish to use.
4. Press OK.
5. Select the variable that you have just created in the Blue Drop-down Menu.

Example

These two correlations are illustrated in File:CorrelationJavaScript.QPack.