certain vs questionable

certain

adj
  • Unfailing; infallible. 

  • Known but not specifically named; indeterminate; indefinite; one or some; sometimes used independently as a noun, and meaning certain persons; see also "one". 

  • Fixed or stated; regular; determinate. 

  • Not to be doubted or denied; established as a fact. 

  • Sure, positive, not doubting. 

  • Actually existing; sure to happen; inevitable. 

det
  • Having been determined but not specified. 

pron
  • Unnamed or undescribed members (of). 

questionable

adj
  • Problematic; open to doubt or challenge. 

  • Of dubious respectability or morality. 

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