How can IDF be different for several documents?

Question

I am using LETOR to make an information retrieval system. They use TF and IDF. I am sure TF is query-dependent. But IDF should be to, but:

"Note that IDF is document independent, and so all the documents under a query have same IDF values."

But that does not make sense because IDF is part of the feature list. How will IDF for each document be calculated?

Answer 1

IDF is term specific. The IDF of any given term is document independent, but the TF is document specific.

To say it differently. Let's say we have 3 documents.

doc id 1 "The quick brown fox jumps over the lazy dog"

doc id 2 "The Sly Fox Pub Annapolis is located on church circle"

doc id 3 "Located on Church Circle, in the heart of the Historic District"

Now if IDF is (number of documents) / (number of documents containing term t) then the IDF for the term fox is 3/2 regardless of what the search is or what the document is. So IDF is a function of t.

TF on the other hand is a funciton on t and d. So the TF of 'the' for doc id 1 is 2.

Answer 2

To add to what jshen said:

IDF is a measure of how common any particular word or gram is in the given corpus that you are searching. It is an estimate of how rare that word is and thus its likely importance. So if a query contains an uncommon word, documents containing that rare word should be judged to be more important.