erratic vs mutable

erratic

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

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

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

  • Anything that has erratic characteristics. 

mutable

adj
  • Changeable, dynamic, evolutive; inclined to change, evolve, mutate. 

  • Having a value that is changeable during program execution. 

  • Being one of the signs Gemini, Virgo, Sagittarius and Pisces, associated with adaptability, flexibility and sympathy. 

noun
  • Something mutable; a variable or value that can change. 

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