'Scores ngrams using Student\'s t test with independence hypothesis
for unigrams, as in Manning and Schutze 5.3.1.'

@classmethod
def student_t(cls, *marginals):

    return ((marginals[NGRAM] - (_product(marginals[UNIGRAMS]) / (marginals[TOTAL] ** (cls._n - 1)))) / ((marginals[NGRAM] + _SMALL) ** 0.5))
