Business Process Models contain a lot of information. They are created with different objectives in mind by people with different background. Many models contain information about an organization’s structure and the application systems or services used within the organization. Several tasks are done by people with different roles using different resources. Identity Management Systems (IDMS) try to offer a way to manage all these information automatically. After introducing an IDMS it is easy to cope with changes in identities (persons) and their roles. However, one main question often remains: How to identify good roles that are capable to ease the task of assigning people to resources? In this article a model-based approach using ratios is presented. Complexity, cohesion and coupling for roles are introduced and evaluated to come to a good set of roles representing what they should represent: a bundle of similar organizational functions and resources.