Who joins a PPO?
D000328:Adult; D000367:Age Factors; D003256:Community Participation; D003657:Decision Making; D003710:Demography; D004651:Employment; D005260:Female; D006279:Health Maintenance Organizations / Q000706:statistics & numerical data; D006801:Humans; D007348:Insurance, Health / Q000706:statistics & numerical data*; D008297:Male; D011243:Preferred Provider Organizations / Q000706:statistics & numerical data*; D012959:Socioeconomic Factors; D014481:United States