discussion vs dissertation

discussion

noun
  • Conversation or debate concerning a particular topic. 

  • Text giving further detail on a subject. 

dissertation

noun
  • A lengthy lecture on a subject; a treatise; a discourse; a sermon. 

  • A formal exposition of a subject, especially a research paper that students write in order to complete the requirements for a doctoral degree in the US and a non-doctoral degree in the UK; a thesis. 

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