keep back vs retain

keep back

verb
  • To prevent (a pupil) from advancing in a course. 

  • To hold back; to refuse to give or share. 

  • To restrict or restrain. 

  • To stay back; to refrain from approaching. 

retain

verb
  • To hold back (a pupil) instead of allowing them to advance to the next class or year. 

  • To employ by paying a retainer. 

  • To keep in one's pay or service. 

  • To keep in possession or use. 

  • To hold secure. 

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