recommendation vs warning

recommendation

noun
  • An act of recommending. 

  • A suggestion or proposal about the best course of action (with adpositions including "about" for the context and "to" for the course of action) 

  • That which is recommended. 

  • A commendation or endorsement. 

warning

noun
  • The action of the verb warn; an instance of warning someone. 

  • Something spoken or written that is intended to warn. 

intj
  • Used to warn of danger in signs and notices. 

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