public function get_aggregate_types() {
        return array (RATING_AGGREGATE_NONE     => get_string('aggregatenone', 'rating'),
                      RATING_AGGREGATE_AVERAGE  => get_string('aggregateavg', 'rating'),
                      RATING_AGGREGATE_COUNT    => get_string('aggregatecount', 'rating'),
                      RATING_AGGREGATE_MAXIMUM  => get_string('aggregatemax', 'rating'),
                      RATING_AGGREGATE_MINIMUM  => get_string('aggregatemin', 'rating'),
                      RATING_AGGREGATE_SUM      => get_string('aggregatesum', 'rating'));
    }