equation vs modus operandi

equation

noun
  • The act or process of equating two or more things, or the state of those things being equal (that is, identical). 

  • An assertion that two expressions are equal, expressed by writing the two expressions separated by an equal sign; from which one is to determine a particular quantity. 

  • A small correction to observed values to remove the effects of systematic errors in an observation. 

modus operandi

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

  • 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. 

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