function getGroupType()
    {
        $this->_createElementsIfNotExist();
        $prevType = '';
        foreach (array_keys($this->_elements) as $key) {
            $type = $this->_elements[$key]->getType();
            if ($type != $prevType && $prevType != '') {
                return 'mixed';
            }
            $prevType = $type;
        }
        return $type;
    }