sole proprietorship vs sole trader

sole proprietorship

noun
  • A business that is wholly owned by a single person, who has unlimited liability. 

sole trader

noun
  • A type of enterprise or proprietorship which is owned by one person who is fully liable for the company's debts and fulfilment of contracts with his/her personal wealth unless incorporated. 

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