def correlations(self):
        """
        Calculate the correlation coefficients.
        """
        corr_df = self._sensors_num_df.corr()
        corr_names = []
        corrs = []

        for i in range(len(corr_df.index)):
            for j in range(len(corr_df.index)):
                c_name = corr_df.index[i]
                r_name = corr_df.columns[j]
                corr_names.append("%s-%s" % (c_name, r_name))
                corrs.append(corr_df.ix[i, j])

        corrs_all = pd.DataFrame(index=corr_names)
        corrs_all["value"] = corrs
        corrs_all = corrs_all.dropna().drop(
            corrs_all[(corrs_all["value"] == float(1))].index
        )
        corrs_all = corrs_all.drop(corrs_all[corrs_all["value"] == float(-1)].index)

        corrs_all = corrs_all.sort_values("value", ascending=False)
        corrs_all = corrs_all.drop_duplicates()
        return corrs_all
Calculate the correlation coefficients.