Calculate Dale-Chall readability score.

def dale_chall(self, diff_count, words, sentences):
        """Calculate Dale-Chall readability score."""
        pdw = diff_count / words * 100
        asl = words / sentences
        raw = 0.1579 * (pdw) + 0.0496 * asl
        if pdw > 5:
            return raw + 3.6365
        return raw