ballot box vs referendum

ballot box

noun
  • The process or method of voting. 

  • A sealed box with a slit, into which a voter puts a completed voting slip. 

referendum

noun
  • A direct popular vote on a proposed law or constitutional amendment. The adposition on is usually used before the related subject of the vote. 

  • An action, choice, etc., which is perceived as passing judgment on another matter. 

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