petty theft vs robbery

petty theft

noun
  • A theft of property having a value lower than a certain small amount set forth in a statute, and considered to be a less serious class of crime. 

robbery

noun
  • The act or practice of robbing. 

  • The offense of taking or attempting to take the property of another by force or threat of force. 

How often have the words petty theft and robbery occurred in a corpus of books? (source: Google Ngram Viewer )