chief minister vs first minister

chief minister

noun
  • The leader of government in many semi-autonomous and constituent federal states. 

first minister

noun
  • The principal minister of a ruler or state. 

  • A leader of a devolved government such as Scotland, Wales, or Northern Ireland. 

  • The prime minister or a provincial or territorial premier. 

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