secret ballot vs voting slip

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. 

voting slip

noun
  • a piece of paper with a list of the candidates at an election (sometimes with an indication of the candidate's party); the voter puts an X in the corresponding place against the candidate of his choice, folds the slip and places it in a ballot box provided. 

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