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Traditional requirements processes often do not address the many problems encountered in the development of software products. Conventional processes begin with the structural definition of the proposed system, under the assumption that the raw requirements are understood. How this understanding is developed is not formally addressed. The IBM software development process requires a methodology to develop the rationale of the requirement, both in terms of its underlying problem and its business justification, prior to the development of the functional specification. Conventional requirements processes address a single software application intended for use by a uniform set of end users. The resulting system is usually a one-time replacement of some existing system. Many IBM software products, however, address requirements received from a large, diverse set of customers who use the products in a wide array of computing environments. Product releases are typically developed as incremental enhancements to an existing base product. This paper describes the Planning and Design Methodology (PDM), a requirements planning process that supports the collection, analysis, documentation, and tracking of software requirements. The process includes requirements collection, definition of the underlying problems, development of an external functional description that addresses the problems, and development of system and product designs from the external functional descriptions. PDM has been applied in three development areas with positive results.
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