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.
in this feature, all the query words jointly contribute through the document score ds, in contrast to the case of PMI where only the pairwise relations are considered.