difficult to believe vs unlikely

difficult to believe

unlikely

adv
  • In an improbable manner. 

noun
  • Something or somebody considered unlikely. 

adj
  • Not holding out a prospect of success; likely to fail; unpromising. 

  • Not likely; improbable; not to be reasonably expected. 

How often have the words difficult to believe and unlikely occurred in a corpus of books? (source: Google Ngram Viewer )