refrain vs slip through the cracks

refrain

verb
  • To stop oneself from some action or interference; to abstain. 

  • To abstain from (food or drink). 

noun
  • The chorus or burden of a song repeated at the end of each verse or stanza. 

  • A much repeated comment, complaint, or saying. 

slip through the cracks

verb
  • To escape notice or lack sufficient attention. 

How often have the words refrain and slip through the cracks occurred in a corpus of books? (source: Google Ngram Viewer )