def basic_stats(self):
        """Return a markdown representation of simple statistics."""
        comment_score = sum(comment.score for comment in self.comments)
        if self.comments:
            comment_duration = (self.comments[-1].created_utc -
                                self.comments[0].created_utc)
            comment_rate = self._rate(len(self.comments), comment_duration)
        else:
            comment_rate = 0

        submission_duration = self.max_date - self.min_date
        submission_rate = self._rate(len(self.submissions),
                                     submission_duration)
        submission_score = sum(sub.score for sub in self.submissions.values())

        values = [('Total', len(self.submissions), len(self.comments)),
                  ('Rate (per day)', '{:.2f}'.format(submission_rate),
                   '{:.2f}'.format(comment_rate)),
                  ('Unique Redditors', len(self.submitters),
                   len(self.commenters)),
                  ('Combined Score', submission_score, comment_score)]

        retval = 'Period: {:.2f} days\n\n'.format(submission_duration / 86400.)
        retval += '||Submissions|Comments|\n:-:|--:|--:\n'
        for quad in values:
            retval += '__{}__|{}|{}\n'.format(*quad)
        return retval + '\n'
Return a markdown representation of simple statistics.