1
general::
Phrase(s): on the average [and] on average
generally; usually. • On the average, you can expect about a 10 percent failure rate. • This report looks OK, on average.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
As a rule, usually, as in On the average, about 15 percent of the freshmen class will drop out before graduation. This expression uses average in the sense of "a norm or standard." [First half of 1700s]
American Heritage Idioms