box office vs observers

box office

noun
  • The total amount of money paid by people worldwide to watch a movie at cinemas/movie theaters. 

  • A place where tickets are sold in a theatre/theater or cinema. 

  • Quality of an entertainment or spectacle that makes it very popular with the public, or likely to be so. 

observers

How often have the words box office and observers occurred in a corpus of books? (source: Google Ngram Viewer )