opinion poll vs referendum

opinion poll

noun
  • A poll organized in an attempt to quantify public opinion on a subject. 

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 opinion poll and referendum occurred in a corpus of books? (source: Google Ngram Viewer )