public function getSimilarity()
    {
        if (count($this->_indices) == 0) {
            include_once 'Zend/Search/Lucene/Exception.php';
            throw new Zend_Search_Lucene_Exception('Indices list is empty');
        }

        $similarity = reset($this->_indices)->getSimilarity();

        foreach ($this->_indices as $index) {
            if ($index->getSimilarity() !== $similarity) {
                include_once 'Zend/Search/Lucene/Exception.php';
                throw new Zend_Search_Lucene_Exception('Indices have different similarity.');
            }
        }

        return $similarity;
    }