CPO vs chief petty officer

CPO

chief petty officer

noun
  • A warrant officer belonging to one of the two classes of chief petty officer: chief petty officer first class and chief petty officer second class. 

  • A non-commissioned officer in the United States Navy and United States Coast Guard, ranking above petty officer first class and below senior chief petty officer. 

  • A non-commissioned officer in the Royal Navy, ranking above petty officer and below warrant officer class 2. 

How often have the words CPO and chief petty officer occurred in a corpus of books? (source: Google Ngram Viewer )