Return the calculated similarity of two
        statements based on the Jaccard index.

def compare(self, statement_a, statement_b):
        """
        Return the calculated similarity of two
        statements based on the Jaccard index.
        """
        # Make both strings lowercase
        document_a = self.nlp(statement_a.text.lower())
        document_b = self.nlp(statement_b.text.lower())

        statement_a_lemmas = set([
            token.lemma_ for token in document_a if not token.is_stop
        ])
        statement_b_lemmas = set([
            token.lemma_ for token in document_b if not token.is_stop
        ])

        # Calculate Jaccard similarity
        numerator = len(statement_a_lemmas.intersection(statement_b_lemmas))
        denominator = float(len(statement_a_lemmas.union(statement_b_lemmas)))
        ratio = numerator / denominator

        return ratio