def mean(self):
        """Compute mean of groups, excluding missing values.

        For multiple groupings, the result index will be a MultiIndex.
        """
        if self._can_use_new_school():
            self._prep_spark_sql_groupby()
            import pyspark.sql.functions as func
            return self._use_aggregation(func.mean)
        self._prep_pandas_groupby()
        return DataFrame.fromDataFrameRDD(
            self._regroup_mergedRDD().values().map(
                lambda x: x.mean()), self.sql_ctx)
Compute mean of groups, excluding missing values.

        For multiple groupings, the result index will be a MultiIndex.