rigour vs severity

rigour

noun
  • Character of being unyielding or inflexible. 

  • A trembling or shivering response. 

  • Higher level of difficulty. 

  • Harshness, as of climate. 

  • Shrewd questioning. 

  • Severity or strictness. 

severity

noun
  • The degree of something undesirable; badness or seriousness. 

  • The state of being severe. 

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