def probabilistic_inverse_document_frequency(term)
      count = @model.document_count(term).to_f
      log((documents.size - count) / count)
    end
def probabilistic_inverse_document_frequency(term)
      count = @model.document_count(term).to_f
      log((documents.size - count) / count)
    end
SMART p, Salton p, Chisholm IDFP