THE MAGIC SQUARE
Three useful levels of refinement are the following:
- The product idea is the most abstract concept of what the product should do. It is comparable in abstraction to a mission statement of an organization.
- A function specification is a description of the function that the product should offer its users.
- A transaction specification is a list of transactions of the product with its users...each...is an interaction with the environment considered to be atomic.
The product objectives specify a problem to be solved; a specification of product objectives is always a specification of user needs.
The specifications of the product idea, product functions and product
transactions describe a solution of the problem at increasing levels of
refinement.
...During refinement, we move from the why to the what. Refinement reduces the uncertainty about a problem situation because we select a solution to a set of design objectives. The movement from right to left decreases the level of refinement
and increases the level of abstraction. During abstraction, we move from the what to the why at the same level of aggregation, i.e. from behavior to function. Because refinement is a decomposition of functions into more detailed functions, it is also called function decomposition...
The movements in the vertical dimension are system decomposition moving down and system integration
moving up. System decomposition moves from the what at one aggregation level to the how of that level (which is the what of the next lower level). Just as refinement resolves uncertainty about the behavior that would satisfy system objectives, system decomposition resolves uncertainty about which decomposition would satisfy system objectives. In functional system decomposition, the levels of decomposition correspond to levels of refinement. There are however other ways to decompose a system and in general, system decomposition is orthogonal to function refinement.
Requirements Engineering, Frameworks for Understanding
By R.J. Wieringa
Copyright 1996 by John Wiley & Sons, Ltd.
Reproduced with permission.