a   b
            tiger  1   2
            zebra  2   3
            >>> df.sem()
            a   0.5
            b   0.5
            dtype: float64

            Using axis=1

            >>> df.sem(axis=1)
            tiger   0.5
            zebra   0.5
            dtype: float64

            In this case, `numeric_only` should be set to `True`
            to avoid getting an error.

            >>> df = pd.DataFrame({'a': [1, 2], 'b': ['T', 'Z']},
            ...                   index=['tiger', 'zebra'])
            >>> df.sem(numeric_only=True)
            a   0.5
            dtype: float64r0