instability vs strength

instability

noun
  • The quality of being unstable. 

  • A state that is not in equilibrium, or in which a small change has a large irreversible effect. 

strength

noun
  • The quality or degree of being strong. 

  • The intensity of a force or power; potency. 

  • The strongest part of something; that on which confidence or reliance is based. 

  • A positive attribute. 

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