How does the Keyword filter on the Search Results page work?

 
How does the Keyword filter on the Search Results page work?
 
 

The Keyword filter includes terms and phrases drawn from the title, index, abstract, and full text fields of the documents included in the user’s search results. The search engine pulls these terms and phrases based on what it believes are the most common topics among the search result set, and then assigns each document to one or more of the extracted Keywords. As the terms and phrases are drawn from the current results set, the Keyword filter is applied across the entire results set. In contrast, the Subjects filter includes editorially assigned subject headings, and not all documents will include editorially assigned subjects, so not all documents in a results set will necessarily be included in this filter. Looked at from this perspective, the Keywords and Subject filters complement one another.