fluency vs utterance

fluency

noun
  • The quality of being fluent in a language; A person's command of a particular language. 

  • The quality of smoothness of flow. 

  • The quality of consistently applying skill correctly in the manner of one well-practiced at it, requiring little deliberate thought to perform without mistakes 

utterance

noun
  • The ability to speak. 

  • Something spoken. 

  • A manner of speaking. 

  • An act of uttering. 

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