prominently throughout the report. This is not to imply that measurements have never been made or that data are totally lacking. Unfortunately metrics tend to describe properties and conditions for which it is easy to gather data rather than those that are useful for characterizing software content, complexity, and form.

PROBLEM FORMULATION AND SPECIFICATION OF REQUIREMENTS

Within the context of system development, specifications for required software functions are derived from the larger system requirements, which are the primary source for determining what the delivered software product will do and how it will do it. These requirements are translated by the designer or design team into a finished product that delivers all that is explicitly stated and does not include anything explicitly forbidden. Some common references regarding requirements specification are mentioned in IEEE Standard for Software Productivity Metrics (IEEE, 1993).

Requirements—the first formal tangible product obtained in the development of a system-are subjective statements specifying the system's various desired operational characteristics. Errors in requirements arise for a number of reasons, including ambiguous statements, inconsistent information, unclear user requirements, and incomplete requests. Projects that have ill-defined or unstated requirements are subject to constant iteration, and a lack of precise requirements is a key source of subsequent software faults. In general, the longer a fault resides in a system before it is detected, the greater is the cost of removing it or recovering from related failures. This condition is a primary driver of the review process throughout software development.

The formulation requirements start with customers requesting a new functionality. Systems engineers collect information describing the new functionality and develop a customer specification description (CSD) describing the customer's view of the feature. The CSD is used internally by software development organizations to formulate cost estimates for bidding. After the feature is committed (sold), systems engineers write a feature specification description (FSD) describing the internal view of the feature. The FSD is commonly referred to as ''requirements." Both the CSD and FSD are carefully reviewed and must meet formal criteria for approval.

DESIGN

The heart of the software development cycle is the translation and refinement of the requirements into code. Software architects transform the requirements for each specified feature into a high-level design. As part of this process, they determine which subsystems (e.g., databases) and modules are required and how they interact or communicate. The broad, high-level design is then refined into a detailed low-level design. This transformation involves much information gathering and detective work. The software architects are often the most experienced and knowledgeable of the software engineers.

The sequence of continual refinements ultimately results in a mapping of high-level functions into modules and code. Part of this design process is selecting an appropriate



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