American football vs association football

American football

noun
  • A game similar to rugby football played on a field of 100 yards long and 53 1/3 yards wide (with two 10 yard-long endzones) in which two teams of 11 players attempt to get an ovoid ball into each other's territory. 

association football

noun
  • A ball game played between two teams of eleven players, each attempting to win by scoring more goals than their opponent. 

How often have the words American football and association football occurred in a corpus of books? (source: Google Ngram Viewer )