rigour vs validity

rigour

noun
  • Severity or strictness. 

  • A trembling or shivering response. 

  • Character of being unyielding or inflexible. 

  • Higher level of difficulty. 

  • Harshness, as of climate. 

  • Shrewd questioning. 

validity

noun
  • State of having legal force. 

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

  • The state of being valid, authentic or genuine. 

  • 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 rigour and validity occurred in a corpus of books? (source: Google Ngram Viewer )