if not vs otherwise

if not

adv
  • Used to link a semantically weaker word to a semantically stronger word following the adverb, both sharing the same basic meaning, indicating the increased likelihood of the former. 

otherwise

adv
  • Differently, in another way. 

  • In different circumstances; or else. 

  • In all other respects. 

adj
  • Other than supposed; different. 

How often have the words if not and otherwise occurred in a corpus of books? (source: Google Ngram Viewer )