affiliation vs membership

affiliation

noun
  • A club, society or umbrella organisation so formed, especially a trade union. 

  • The relationship resulting from affiliating one thing with another. 

  • The establishment of a child's paternity or maternity 

membership

noun
  • The state of being a member of a group or organization. 

  • The body of members of an organization. 

  • The fact of being a member of a set. 

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