def inverse_document_frequency(term)
      df = @model.document_count(term)
      log((documents.size - df + 0.5) / (df + 0.5))
    end
def inverse_document_frequency(term)
      df = @model.document_count(term)
      log((documents.size - df + 0.5) / (df + 0.5))
    end
Return the term's inverse document frequency.

 @param [String] term a term
 @return [Float] the term's inverse document frequency