ethnic group vs phratry

ethnic group

noun
  • A group of people who identify with one another, especially on the basis of ancestral, national, cultural, historical or religious grounds. 

phratry

noun
  • A clan or kinship group consisting of a number of families claiming descent from a common ancestor and having certain collective functions and responsibilities. 

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