competency vs skill set

competency

noun
  • The ability to perform some task; competence. 

  • Implicit knowledge of a language’s structure. 

  • Meeting specified qualifications to perform. 

skill set

noun
  • The skills needed to accomplish a specified task or perform a given function. 

  • A set of skills. 

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