equation vs proportionality

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. 

proportionality

noun
  • the degree to which something is in proportion 

  • the property of being proportional 

  • the principle that government action ought to be proportional to the ends achieved (e.g. the military should not be deployed to stop petty vandalism) 

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