function variance( arr ) {
	var delta;
	var mean;
	var len;
	var M2;
	var i;
	var x;

	delta = 0.0;
	mean = 0.0;
	M2 = 0.0;
	len = arr.length;
	for ( i = 0; i < len; i++ ) {
		x = arr[ i ];
		delta = x - mean;
		mean += delta / (i+1);
		M2 += delta * ( x - mean );
	}
	return M2 / ( i - 1 );
}
Computes the unbiased variance of an array.

@private
@param {NumericArray} arr - input array
@returns {number} variance