periodically vs rarely

periodically

adv
  • intermittently or recurrently 

  • In a manner that is regular or periodic. 

rarely

adv
  • Not occurring at a regular interval; seldom; not often. 

  • To a rare degree; very. 

  • Unusually well; excellently. 

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