add up vs knock on wood

add up

verb
  • To make sense; to be reasonable or consistent. 

  • To take the sum of; to total. 

  • To accumulate; to amount to. 

knock on wood

verb
  • To take a customary action to ward off some misfortune that is believed to be attracted by a presumptuous statement. 

intj
  • Hopefully; a self-directive to undertake the customary action to ward off bad luck. 

How often have the words add up and knock on wood occurred in a corpus of books? (source: Google Ngram Viewer )