function absMeanDiff( x, groups, levels ) {
	var accumulators = {};
	var means = {};
	var len = x.length;
	var out = [];
	var i;
	for ( i = 0; i < levels.length; i++ ) {
		accumulators[ levels[ i ] ] = incrmean();
	}
	for ( i = 0; i < len; i++ ) {
		accumulators[ groups[ i ] ]( x[ i ] );
	}
	for ( i = 0; i < levels.length; i++ ) {
		means[ levels[ i ] ] = accumulators[ levels[ i ] ]();
	}
	for ( i = 0; i < len; i++ ) {
		out.push( abs( x[ i ] - means[ groups[ i ] ] ) );
	}
	return out;
}
Calculates the absolute difference of the values and the respective group means for the elements of a numeric array.

@private
@param {Array} x - input array
@param {Array} groups - array of group labels
@param {Array} levels - array of distinct group levels
@returns {Array} array of absolute centered values

@example
var x = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ];
var groups = [ 'a', 'b', 'a', 'a', 'b', 'b', 'a', 'a', 'b', 'b' ];
var levels = [ 'a', 'b' ];
var out = absMeanDiff( x, groups, levels );
// returns [ ~3.6, 4.4, ..., ~3.6 ]