Mark as 'enriched' or 'purified'.

def _init_enrichment(self):
        """Mark as 'enriched' or 'purified'."""
        if self.study_n:
            return 'e' if ((1.0 * self.study_count / self.study_n) >
                           (1.0 * self.pop_count / self.pop_n)) else 'p'
        return 'p'