believability vs validity

believability

noun
  • The state or quality of being believable. 

validity

noun
  • The state of being valid, authentic or genuine. 

  • The genuinity - as distinguished from the efficacity or the regularity - of a sacrament as a result of some formal dispositions being fulfilled. 

  • State of having legal force. 

  • A quality of a measurement indicating the degree to which the measure reflects the underlying construct, that is, whether it measures what it purports to measure (see reliability). 

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