criticism vs flak

criticism

noun
  • The act of criticising; a critical judgment passed or expressed 

  • A critical observation or detailed examination and review. 

flak

noun
  • Adverse criticism. 

  • A public-relations spokesperson. 

  • Anti-aircraft shell fire. 

  • Ground-based anti-aircraft guns firing explosive shells. 

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