confusion vs knotty question

confusion

noun
  • The act of mistaking one thing for another or conflating distinct things. 

  • A lack of clarity or order. 

  • The state of being confused; misunderstanding. 

  • Lack of understanding due to dementia. 

knotty question

How often have the words confusion and knotty question occurred in a corpus of books? (source: Google Ngram Viewer )