Secondary Sources are «works including books, journal articles, speeches, reviews, research reports that analyze, assess or interpret an historical event, era, or phenomenon, by someone who did not experience first-hand or participate in the events or conditions, and generally utilizing primary sources to do so. Secondary sources often offer a review or a critique. Also included would be reference sources like encyclopedias» (Getty Art and Architecture Thesaurus - Getty AAT).
The Secondary Sources entity includes works that were publiched after 1911 and the closing of the Fair, by someone who did not attend the event, and who used primary sources to discuss the Fair. The records are subdivided into articles, books, and chapters (layout features).