erratic vs invariable

erratic

noun
  • Anything that has erratic characteristics. 

  • A rock moved from one location to another, usually by a glacier. 

adj
  • Deviating from normal opinions or actions; eccentric; odd. 

  • Unsteady, random; prone to unexpected changes; not consistent. 

invariable

noun
  • Something that does not vary; a constant. 

adj
  • Constant. 

  • Not variable; unalterable; uniform; always having the same value. 

  • That cannot undergo inflection, conjugation or declension. 

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