ballot paper vs secret ballot

ballot paper

noun
  • A voting form; the paper upon which a vote is cast during a ballot. 

secret ballot

noun
  • The voting system involving ballots available only at official polling places, prepared at public expense, containing the names of all candidates, and marked in secret at the polling places. 

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