I was interested in learning about the algorithms that tries to match document and our queries when it tries to match with the documents and returns on a rank from best match to least match. In this blog I have tried to figure out the question why I have to use tf-idf and cosine similarity .

Jaccard similarity coefficient is too straight forward. For a specific search query It figures out the rank depending on the word it has matched in the total number of unique word it has. I think rather than saying like this, if I say formally then it would be a lot more clear. formally it is defined by the division of cardinality of the set of their intersection and the cardinality of the set of unions.

(1) \begin{equation*} Jaccard(A,B)= \frac{ | A \cup B |}{ | A \cap B |} \end{equation*}

Example:

query=ides o march

document1=caesar died in march

document2=the long march

jaccard(query, document1)=1/6

jaccard(query, document2)=1/5

But Jaccard is too binary to be awesome. In real life if a word frequently appears in a document then the other than the query should retrieval the first document instead of the second one but jaccard coefficient does not count the frequency at all, it checks whether the word appears or not, it is boolean. The problem with Boolean queries are that, it often results too many or too few results. When we use AND it becomes too short and for OR the results become too short. So for long query it becomes too choosy and for short queries it shows everything that matches.

We have detected a problem, so now it is time to solve it. We will try to solve it using Term Frequency Waiting. This time we will try to move away from boolean approach. So this time a presence of a word in a document is not enough but their count is also something of our headache. So the same thing of above but this time we will count the words it has appeared. Suppose for the sentence: “in the month of march, they are going for long march” will vote twice for the document when the query is “march”, in fact it will be 2/11.

tf(query, document)= total query words present in document/ total number of words in query and document

Let me remind the scenario once again so that we can detect any possible bug in our assumption, suppose I am looking for a word and one document has it once and the other document has it 10 times. obviously the 10 time will be the one I would more likely be looking for but it won’t be 10 time more important. So we can use log assumption here. So we can do,

(2) \begin{equation*} w= 1+log(tf) \end{equation*}

But guess what? what is the most used word in our conversation? I am pretty sure that “is, are, it” sort of word will get the most frequency rate but it appears in all the documents which won’t help us to do anything at all. In fact we are interested about rare words – the antique one.

So it was the necessity for introducing Inverse Frequency Model. So let me tell you again the rare terms are more important here for now. For example a document full of the, to is not important to recognize the document anymore because it appears in every document. So in this model the more it appears in the documents, it is less important… in other word the important get divided. So lets do it,

(3) \begin{equation*} idf=\frac{N}{df} \end{equation*}

where N= total number of documents we have,

df= the number of document that has our query word

But again it does not increase like a plain line, so lets use logarithmic scale for the same reason as above.

(4) \begin{equation*} idf=Log \frac{N}{df} \end{equation*}

more formally:

(5) \begin{equation*} idf(t,D)= log(tf) \end{equation*}

so now for 1 occurance in 1M will get idf = 6

TF only care about the number of time it appears in the document and idf cares about the rare words. But we need a proper balance of things. It should be rare but at the same time that rare word should appear multiple time in the document. So how would we balance? we can just multiply both of them. tf-idf is the best weighting scheme known.

Let me help you a little bit more with its implementation. When you are done your tf-idf would look like a 1xm vector matrix. somewhat like array.

[It, is, March]=[0.33,0.33,0]

Remember how do we draw vectors? if a vector is represented as [x,y] just ploting line that go through [0,0] and that connects [x,y]. TF-IDF makes it multidimensional nothing else. we have represented something using tf-idf, thats cool, but how would we we retrive ranks? We can make tf-idf matrix for both query and document then calculate vector distance. But here we can face a new problem because of its vector nature. remember that [2,2] and [4,4] represent the same line, just twice as large as the line. Which is a drawback of this tf-idf because it adds the distance between query and document very high. So to get out of this draw back we will introduce cosine similarity. Basically cosine similarity is pretty simple idea, instead of distance now we will calculate the angle between the lines.

(6) \begin{equation*} cos(\theta)=\frac{A \dot B}{| A | | B |} \end{equation*}

so from angle now we can detect the similarity.