doubtful vs nonconvincing

doubtful

adj
  • Unclear or unreliable. 

  • Experiencing or showing doubt, sceptical. 

  • Subject to, or causing doubt. 

  • Improbable or unlikely. 

  • Suspicious, or of dubious character. 

  • Undecided or of uncertain outcome. 

noun
  • A doubtful person or thing. 

nonconvincing

adj
  • unconvincing 

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