credible vs unbelievable

credible

adj
  • Believable or plausible. 

  • Authentic or convincing. 

  • Dependable or reliable. 

unbelievable

adj
  • Implausible or improbable. 

  • Not to be believed. 

  • Amazing, incredible; so surprising it is almost unable to believe. 

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