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about HMOs then than we had ten years previously.35 The reason? As HMOs grew, they diversified. Initially, there were two dominant models: Staff-Model HMOs that employed a closed network of physicians that they employed; and Group-Model HMOs that had exclusive contracts with two or more physician groups. Independent Practice Association (IPA) HMO –the renamed Foundation Medical Plans that emerged at the time of PPGPs– had the smallest market share. IPA- HMOs contracted with self-employed physicians or group practices, or intermediary groups that contacted with self-employed physicians. Physicians could continue to treat patients covered by other insurers. They could join or leave the network based on local conditions. These features allowed IPAs to vary their contractual terms based on regional differences and changing That allowed them to rapidly expand networks without incurring fixed employment costs. They eventually became the dominant model HMO. HMOs, particularly –IPA-HMOs– developed widely different methods of finance, provider payment, contracts, utilization review, practice guidelines, and gate keeping.36 As HMOs diversified, it was no longer possible to generalize about their organization or function; it became almost impossible to draw general conclusions about HMO performance.37 Here are some key ways HMOs changed.38 s The market share grew for IPA-HMOs, which created broad provider networks by contracting with self-employed physicians and independent hospitals. That led to declining market shares by integrated HMOs; i.e., those offering limited provider networks, employing physicians, and owning facilities. s HMOs increasingly moved from directly contracting with physicians to contracting with intermediaries such as IPAs