normally vs on average

normally

adv
  • In the manner of a variable with a Gaussian distribution. 

  • Under normal conditions or circumstances; usually; most of the time 

  • To a usual or customary extent or degree. 

  • In the expected or customary manner. 

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