need vs ought to

need

verb
  • To be obliged or required (to do something). 

  • To be required; to be necessary. 

  • To want strongly; to feel that one must have something. 

  • To have an absolute requirement for. 

noun
  • Lack of means of subsistence; poverty; indigence; destitution. 

  • A requirement for something; something needed. 

ought to

verb
  • Indicating duty or obligation. 

  • Indicating advisability or prudence. 

  • Indicating desirability. 

  • Indicating likelihood or probability. 

How often have the words need and ought to occurred in a corpus of books? (source: Google Ngram Viewer )