sorority vs trade union

sorority

noun
  • A group of girls or women associated for a common purpose; a sisterhood. 

  • A social organization of female students at a college or university; usually identified by Greek letters. 

trade union

noun
  • An organization whose members belong to the same trade and that acts collectively to address common issues. 

  • labour union (Canada), labor union (North America) 

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