foster parent vs nurse practitioner

foster parent

noun
  • An adult who cares for a child who has been placed in foster care. 

nurse practitioner

noun
  • A professional nursing designation recognized in some jurisdictions, denoting an individual who is fully qualified as a nurse and who has additional qualifications and authorization to provide diagnoses and/or treatments, including writing prescriptions . 

How often have the words foster parent and nurse practitioner occurred in a corpus of books? (source: Google Ngram Viewer )