'Return a summary of the estimated survival function.
The summary is a datafram containing the unique event times,
estimated survival function values, and related quantities.'

def summary(self):

    df = pd.DataFrame(index=self.surv_times)
    df.index.name = 'Time'
    df['Surv    prob'] = self.surv_prob
    df['Surv    prob    SE'] = self.surv_prob_se
    df['num    at    risk'] = self.n_risk
    df['num    events'] = self.n_events
    return df
