PM vs head of government

PM

verb
  • To project manage. 

  • To send a private message to (a participant in a chat room, etc.). 

head of government

noun
  • The chief officer of the executive branch of a government, often presiding over a cabinet; usually called Prime Minister (in a parliamentary system) or President (in a presidential system). 

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