in which an instance of either entity can be related to any number of instances of the other. View levels Three levels of view are defined in IDEF1X: entity relationship (ER), key-based (KB), and fully attributed (FA). They differ in level of abstraction. The ER level is the most abstract. It models the most fundamental elements of the subject area - the entities and their relationships. It is usually broader in scope than the other levels. The KB level adds keys and the FA level adds all the attributes. IDEF1X topics The three schema approach The three-schema approach in software engineering is an approach to building information systems and systems information management, that promotes the conceptual model as the key to achieving data integration. A schema is a model, usually depicted by a diagram and sometimes accompanied by a language description. The three schemas used in this approach are: External schema for user views Conceptual schema integrates external schemata Internal schema that defines physical storage structures. At the center, the conceptual schema defines the ontology of the concepts as the users think of them and talk about them. The physical schema describes the internal formats of the data stored in the database, and the external schema defines the view of the data presented to the application programs. The framework attempted to permit multiple data models to be used for external schemata. Modeling guidelines The modeling process can be divided into five stages of model developing. Phase zero – project Initiation The objectives of the project initiation phase include: Project definition – a general statement of what has to be done, why, and how it will get done Source material – a plan for the acquisition of source material, including indexing and filing Author conventions – a fundamental declaration of the conventions (optional methods) by which the author chooses to make and manage the model. Phase one – entity definition The objective of the entity definition phase is to identify and define the entities that fall within the problem domain being modeled. Phase two – relationship definition The objective of the relationship definition phase is to identify and define the basic relationships between entities. At this stage of modeling, some relationships may be non-specific and will require additional refinement in subsequent phases. The primary outputs from phase two are: Relationship matrix Relationship definitions Entity-level diagrams. Phase three - key definitions The