Pointwise mutual information and tf idf, when to use them

When creating a network from a text or a corpus of texts, the logic is to draw the terms that frequently appear (“co-occur”) together in the same sentences or paragraphs. The problem is, most frequent terms in these texts will be connected even if these connections don’t make a lot of sense.

The problem with co-occurrences: lots of noise

For example, we can expect words such as “the”, “and”, “but”… to be frequently used in any text in English. Just because they are so frequent, they will appear frequently together as well.

As a result, when building a network of co-occurring terms, these very common words will be strongly connected and appear prominently in the network. We would have preferred something else:

  • terms that co-occur should be connected, yes
  • but meaningful connections are those between unfrequent terms that co-occur.

Example :

  • we don’t care that “but” / “the” / “or”… appear frequently in the same sentence, because that is to be expected, given that they are so common throughout the text.
  • but we do care that in a text about world capitals for instance, “Paris” and “France”, though they appear unfrequently in the text, keep occurring frequently in the same sentences.

How to automagically build a network that takes this nuance into account? Simple: for a pair of terms (“Paris”, “France”), divide the number of time they co-occur by the total number of time each term appears in the text:

PMI of a cooccurrence between A and B = count of the coocurrences between A and B ÷ (total count of occurrences of A + total count of occurrences of B)

This is one simple and powerful variant of “Pointwise Mutual Information” and it is explained very simply below:

Pointwise mutual information: the solution to discount edges with low information

Let’s imagine a very long text discussing all the world capitals:

  • in 12 sentences, “Paris” and “France” are both mentioned,
  • “Paris” appears 15 times in the entire text,
  • “France” appears 14 times in the entire text,

-> it means that Paris and France appear almost always in the same sentences (they “co-occur”), and very rarely in different sentences! The weight of the edge between them is 12, but with the reasoning above it can be corrected to:

12 ÷ (15 + 14) = 0.41

Meanwhile, in the same text about world capitals, let’s imagine that the words “but” and “if” appear in the same sentences 72 times. Not because these terms are so important in a text on world capitals, but simply because they are so common in the English language. Without a correction, we would find a big edge (weight = 72!) linking the two, bigger than the connection between Paris and France! (weight = 12).

Now let’s correct:

“but” and “if” co-occur 72 times in the text (there are 72 sentences that include both “but” and “if”),

  • “but” appears 350 times in the text,
  • “if” appears 170 times in the text,

-> it means that though “if and “but” appear frequently together in the text, they also appear many more times separately in the text as well. The weight of the edge between them is 72, but with the reasoning above it can be corrected to:

72 ÷ (350 + 170) = 0.13

-> with the correction, the pair “Paris, France” (strength: 0.41) is now stronger than “but, if” (strength: 0.13). Applied to all pairs of the network, this correction will lead to a much more informative result.

What we did here is using a variant of the method called Pointwise Mutual Information. The Wikipedia entry for PMI is pretty complex but explains the same principles I presented here.

An example

I used a variant of PMI in a study of the scientific articles in the field of neuroethics from 1995 to 2012, which amounted to 1925 documents.

We first categorized each article according to one or several broad subject categories they were discussing (legal studies, neuroimaging, philosophy of mind, etc.).

Then, we wondered: which subject categories “co-occur”, meaning: which subject categories tend to be often discussed together (by the same papers). If I did not apply the PMI correction, then the most freqent subject categories would have been associated, and the less frequent ones would have had weak associations simply because of this frequency effect, hiding the fact that subject categories rarely discussed in articles actually tend to be discussed in the same papers.

The PMI correction helped us reveal these less frequent connections, but which were meaningful:

A network of co-occurring subject categories corrected with a PMI method

What is the difference between Pointwise Mutual Information (PMI) and TF-IDF?

You might have heard about TF-IDF, which is a famous method in information retrieval, and quite similar in goal to the PMI I just described.

The common goal between PMI and TF-IDF is to remove this bias (or fallacy): something is not meaningful because it is observed frequently somewhere, if it is just the side effect of this thing being frequently present everywhere.

  1. For PMI: a co-occurrence between A and B is not very meaningful if A and B keep occurring separately as well.

  2. For TF-IDF: a word “A” being used a lot in a document…

    • does not mean that this document is the most relevant about “A”, because “A” might just be a word commonly used in many, many documents (not just this one)
    • does not mean that “A” is the key topic or focus on this document: again, “A” might be frequent in this document for the simple reason that this is a general word used frequently everywhere, in all documents…

The same way PMI tells us how special a co-occurrence really is, TF-IDF tells us how special a word A is for a document where it appears. The wikipedia page for TF-IDF is again quite intimidating, but a basic and working version is:

TF-IDF for A in a document = count of A in this document ÷ count of documents that contain A

(the higher the TF-IDF measure, the most specific and “key” is A in this document)

In the same paper mentioned above, we used a variant of TF-IDF to measure what were the key terms in this or that subject category in neuroethics. When one term is occurring a lot in a category, does it mean this term is the most “typical” of this category? Maybe that she is just a very common term used in the scientific discourse, or in neuroethics broadly! The TF-IDF measure helped establish how “specific” a term was for each subject category.

Want to use Pointwise mutual information to improve your networks?

Nocode functions offers a function which turns texts into networks. It includes a PMI correction which is slightly more agressive than the one presented above. It does:

Corrected weight = weight of cooccurrence (A,B) ÷ (count of A x count of B).

(so the only thing we did as compared to the example above is to use a multiplication instead of an addition).

Questions? Ping me on Twitter!

Written on August 30, 2021