def METHOD_NAME(self, x):
    """Probability density function."""
    d = self.loc[x.index, x.columns]
    pdf_arr = np.exp(-0.5 * ((x.values - d.mu) / d.sigma) ** 2)
    pdf_arr = pdf_arr / (d.sigma * np.sqrt(2 * np.pi))
    return pd.DataFrame(pdf_arr, index=x.index, columns=x.columns)