8Bachman, Keith H., and Donald K. Freeborn, 1999. "HMO Physicians' Use of Referrals."Social Science & Medicinc.48: 547-57.
9Propper, Carol, Deborah Wilson and Neil Soderlund, 1998.“The Effects of Regulation and Competition in the NHS Market: the Case of General Practice Fundholder Prices.”Journal of Health Economics. 17(6): 645-73.