def var(self, ddof=1):
        """Compute standard deviation of groups, excluding missing values.

        For multiple groupings, the result index will be a MultiIndex.
        """
        self._prep_pandas_groupby()
        return DataFrame.fromDataFrameRDD(
            self._regroup_mergedRDD().values().map(
                lambda x: x.var(ddof=ddof)), self.sql_ctx)