mostly vs on average

mostly

adv
  • Mainly or chiefly; for the most part; usually, generally, on the whole. 

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 mostly and on average occurred in a corpus of books? (source: Google Ngram Viewer )