The document score ds is defined by Term Frequency-Inverse Document Frequency (TF-IDF), which is written as where tf (w ′ , d) is the frequency of the word w ′ in the document d, idf (w ′ ) is the inverse document frequency of the word w ′ , and ℓ(d) is the length of d. The VQA feature uses document-local statistics (except for IDF) and counts only in the topk search results.
and  demonstrated that the system performance was boosted by multi-step inference that combines, e.g., taxonomic knowledge ("N.Y. is in the northern hemisphere") and general law ("The summer solstice in the northern hemisphere is in June").