 An HMO may also contract with an existing, independent group practice ("independent group model"), which will generally continue to treat non-HMO patients.Group model HMOs are also considered closed-panel, because doctors must be part of the group practice to participate in the HMO - the HMO panel is closed to other physicians in the community.If not already part of a group medical practice, physicians may contract with an independent practice association (IPA), which in turn contracts with the HMO.