Review miningN-gram
A run of consecutive words extracted from text. How keyword candidates are pulled out of titles and reviews.
Two-word (bigram) and three-word (trigram) phrases are the lengths closest to search behaviour. Single words tend to be too general; four words and up is a sentence.
N-grams from rival titles give you the category's shared vocabulary. N-grams from reviews give you the user's own language - and the second is the valuable one.
Reviews need a filter: people write sentences. Phrases carrying a first or second person pronoun (“it helped me a lot”) are frequent, they even show up in rankings, and nobody types them into a search box.