Compute the expected lifetime, E[T], using covariates X.

        Parameters
        ----------
        X: a (n,d) covariate numpy array or DataFrame
            If a DataFrame, columns
            can be in any order. If a numpy array, columns must be in the
            same order as the training data.

        Returns the expected lifetimes for the individuals

def predict_expectation(self, X):
        """
        Compute the expected lifetime, E[T], using covariates X.

        Parameters
        ----------
        X: a (n,d) covariate numpy array or DataFrame
            If a DataFrame, columns
            can be in any order. If a numpy array, columns must be in the
            same order as the training data.

        Returns the expected lifetimes for the individuals
        """
        index = _get_index(X)
        t = self._index
        return pd.DataFrame(trapz(self.predict_survival_function(X)[index].values.T, t), index=index)