chiefly vs on average

chiefly

adj
  • of or relating to a chief 

adv
  • Especially or primarily; above all. 

  • Mainly or principally; almost entirely. 

on average

prep
  • Expressed as a measure of central tendency (usually the arithmetic mean). 

  • Usually, typically; as a rule; as often as not. 

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