public static function getConversionGroupUnits($group = NULL) {
		$conversionGroups = array();
		foreach(self::$_conversionUnits as $conversionUnit => $conversionGroup) {
			if ((is_null($group)) || ($conversionGroup['Group'] == $group)) {
				$conversionGroups[$conversionGroup['Group']][] = $conversionUnit;
			}
		}
		return $conversionGroups;
	}