modus operandi vs stepping-stone

modus operandi

noun
  • A known criminal's established habits and mode of work when committing specific offences, especially fraud, matched with characteristics of an unsolved crime to narrow down (limit to a specific list) or profile suspects. 

  • A person or thing's normal mode of operation. 

stepping-stone

How often have the words modus operandi and stepping-stone occurred in a corpus of books? (source: Google Ngram Viewer )