function apcorr( arr, mx, my, bool ) {
	var cov;
	var sd;
	if ( bool === false && arr.length < 2 ) {
		return 0.0;
	}
	sd = stdev( [ 0.0, 0.0 ], arr, mx, my, bool );
	cov = covariance( arr, mx, my, bool );
	return abs( cov / ( sd[0]*sd[1] ) );
}
Computes the sample absolute Pearson product-moment correlation coefficient using textbook formula.

@private
@param {ArrayArray} arr - input array
@param {number} mx - `x` mean
@param {number} my - `y` mean
@param {boolean} bool - boolean indicating whether to compute the population correlation coefficient
@returns {number} absolute correlation coefficient