Parameters
        ----------
        Fit the Aalen Additive model to a dataset.

        Parameters
        ----------
        df: DataFrame
            a Pandas DataFrame with necessary columns `duration_col` and
            `event_col` (see below), covariates columns, and special columns (weights).
            `duration_col` refers to
            the lifetimes of the subjects. `event_col` refers to whether
            the 'death' events was observed: 1 if observed, 0 else (censored).

        duration_col: string
            the name of the column in DataFrame that contains the subjects'
            lifetimes.

        event_col: string, optional
            the  name of the column in DataFrame that contains the subjects' death
            observation. If left as None, assume all individuals are uncensored.

        weights_col: string, optional
            an optional column in the DataFrame, df, that denotes the weight per subject.
            This column is expelled and not used as a covariate, but as a weight in the
            final regression. Default weight is 1.
            This can be used for case-weights. For example, a weight of 2 means there were two subjects with
            identical observations.
            This can be used for sampling weights.

        show_progress: boolean, optional (default=False)
            Since the fitter is iterative, show iteration number.


        Returns
        -------
        self: AalenAdditiveFitter
            self with additional new properties: ``cumulative_hazards_``, etc.

        Examples
        --------
        >>> from lifelines import AalenAdditiveFitter
        >>>
        >>> df = pd.DataFrame({
        >>>     'T': [5, 3, 9, 8, 7, 4, 4, 3, 2, 5, 6, 7],
        >>>     'E': [1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 0],
        >>>     'var': [0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2, 2],
        >>>     'age': [4, 3, 9, 8, 7, 4, 4, 3, 2, 5, 6, 7],
        >>> })
        >>>
        >>> aaf = AalenAdditiveFitter()
        >>> aaf.fit(df, 'T', 'E')
        >>> aaf.predict_median(df)
        >>> aaf.print_summary()

def fit(self, df, duration_col, event_col=None, weights_col=None, show_progress=False):
        """
        Parameters
        ----------
        Fit the Aalen Additive model to a dataset.

        Parameters
        ----------
        df: DataFrame
            a Pandas DataFrame with necessary columns `duration_col` and
            `event_col` (see below), covariates columns, and special columns (weights).
            `duration_col` refers to
            the lifetimes of the subjects. `event_col` refers to whether
            the 'death' events was observed: 1 if observed, 0 else (censored).

        duration_col: string
            the name of the column in DataFrame that contains the subjects'
            lifetimes.

        event_col: string, optional
            the  name of the column in DataFrame that contains the subjects' death
            observation. If left as None, assume all individuals are uncensored.

        weights_col: string, optional
            an optional column in the DataFrame, df, that denotes the weight per subject.
            This column is expelled and not used as a covariate, but as a weight in the
            final regression. Default weight is 1.
            This can be used for case-weights. For example, a weight of 2 means there were two subjects with
            identical observations.
            This can be used for sampling weights.

        show_progress: boolean, optional (default=False)
            Since the fitter is iterative, show iteration number.


        Returns
        -------
        self: AalenAdditiveFitter
            self with additional new properties: ``cumulative_hazards_``, etc.

        Examples
        --------
        >>> from lifelines import AalenAdditiveFitter
        >>>
        >>> df = pd.DataFrame({
        >>>     'T': [5, 3, 9, 8, 7, 4, 4, 3, 2, 5, 6, 7],
        >>>     'E': [1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 0],
        >>>     'var': [0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2, 2],
        >>>     'age': [4, 3, 9, 8, 7, 4, 4, 3, 2, 5, 6, 7],
        >>> })
        >>>
        >>> aaf = AalenAdditiveFitter()
        >>> aaf.fit(df, 'T', 'E')
        >>> aaf.predict_median(df)
        >>> aaf.print_summary()

        """
        self._time_fit_was_called = datetime.utcnow().strftime("%Y-%m-%d %H:%M:%S") + " UTC"
        self._censoring_type = CensoringType.RIGHT

        df = df.copy()

        self.duration_col = duration_col
        self.event_col = event_col
        self.weights_col = weights_col

        self._n_examples = df.shape[0]

        X, T, E, weights = self._preprocess_dataframe(df)

        self.durations = T.copy()
        self.event_observed = E.copy()
        self.weights = weights.copy()

        self._norm_std = X.std(0)

        # if we included an intercept, we need to fix not divide by zero.
        if self.fit_intercept:
            self._norm_std["_intercept"] = 1.0
        else:
            # a _intercept was provided
            self._norm_std[self._norm_std < 1e-8] = 1.0

        self.hazards_, self.cumulative_hazards_, self.cumulative_variance_ = self._fit_model(
            normalize(X, 0, self._norm_std), T, E, weights, show_progress
        )
        self.hazards_ /= self._norm_std
        self.cumulative_hazards_ /= self._norm_std
        self.cumulative_variance_ /= self._norm_std
        self.confidence_intervals_ = self._compute_confidence_intervals()

        self._index = self.hazards_.index

        self._predicted_hazards_ = self.predict_cumulative_hazard(X).iloc[-1].values.ravel()
        return self