Refinement

Stepwise refinement is a top-down design strategy initially proposed by Nicklaus Wirth. The architecture of a program is established through successively refining stages of procedural detail. The hierarchy is established through decomposing a macroscopic statement of function a procedural abstraction in a step wise fashion until programming language statements are reached. By an overview of the concept is provided by Wirth:

In the each step of the refinement 1 or various instructions of the given program are decomposed into more detailed instructions. This successive refinement or decomposition of specifications terminates when all orders are expressed in terms of any underlying computer or programming language. As tasks are refined so the data may have to be decomposed, refined, or structured and it is natural to refine the program and the data specifications in parallel.

Every modification step implies some design decisions. That is important the programmer be aware of the underlying criteria for design decisions and of the existence of option solutions.

The procedure of program refinement proposed through Wirth   is analogous to the procedure of partitioning and refinement which is used during requirements analysis. The dissimilarity is in the level of implementation detail which is considered not the approach.

The Refinement is actually a process of amplification. We start with a statement of function or description of information which is described at a high level of abstraction. That is the statement function or information conceptually but provides no information about the internal workings of the function or the internal structure of the information. The Refinement causes the designer to elaborate on the real statement, providing more and more detail as each successive refinement elaboration happen.

The Abstraction and refinement are complementary concepts.  The Abstraction enables a designer to specify process and data and since suppress low-level details. The Refinement helps the designer to reveal low-level details as design progresses. Both concepts aid the designer in established a complete design model as the design evolves.