The key themes by tf and tf-idf

We can look at the key themes in each topic by measuring the term-frequency and tf-idf[^20].

tf–idf or TFIDF, short for term frequency–inverse document frequency, is a numerical statistic that is intended to reflect how important a word is to a document in a collection or corpus.

[^20:] https://en.wikipedia.org/wiki/Tf-idf Accessed 10 September, 2019.