Positive pointwise mutual information replaces all negative PMI values with zero:
$$PPMI(w, c) = \max \left( \log_2(\frac{P(w, c)}{P(w)P(c)}), 0 \right)$$

University of Michigan - Ann Arbor

The Positive Pointwise Mutual Information (PPMI) Algorithm is an algorithm for weighting term-term matrices.

Positive Pointwise Mutual Information (PPMI)

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

Speech and Language Processing (3rd ed. draft) 

The PPMI vector models can be used to compute similarities between words. 

For example, we can find the 10 most similar words to any target word $$w$$ by computing the cosines between $$w$$ and each of the $$V − 1$$ other words, sorting, and looking at the top 10.


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