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水平有限，如有错误请指正！
tf-idf weighting
tf(term frequency)
a document or zone that mentions a query term more often has more to do with that query and therefore should receive a higher score
query term: a set of words
NN:document中term的总数
tt:document中term出现的次数
tft=tN
tf_t=frac{t}{N}
idf(inverse document frequency)
NN: document的数量
dftdf_t:有多少documents 包含tt
idft=logNdft
idf_t=logfrac{N}{df_t}
从公式可以看出：
- dftdf_t 越小，idftidf_t越大,表明tt对文档的区分性更大
- 反之
tf-idf weighting
tf-idft,d=tft,d∗idft
text{tf-idf}_{t,d}=tf_{t,d}* idf_t
参考：
tf
idf
tf-idf
