Master of Science vs master's degree

Master of Science

noun
  • A degree that usually requires 2-3 years beyond a Bachelor of Science and emphasizes science and scientific applications. The degree is less advanced than a Doctorate degree. 

master's degree

noun
  • A postgraduate degree which aims to assist students in developing a mastery for a particular academic field or profession. 

How often have the words Master of Science and master's degree occurred in a corpus of books? (source: Google Ngram Viewer )