Software Engineerig-Observations on Estimating

A leading executive was once asked what single characteristic was most important when selecting a project manager. His response: "a person with the ability to know what will go wrong before it actually does . . ." We might add: "and the courage to estimate when the future is cloudy."

Estimation of resources, cost, and schedule for a software engineering effort requires experience, access to good historical information, and the courage to commit to quantitative predictions when qualitative information is all that exists. Estimation carries inherent risk and this risk leads to uncertainty.

Project complexity has a strong effect on the uncertainty inherent in planning. Complexity, however, is a relative measure that is affected by familiarity with past effort. The first-time developer of a sophisticated e-commerce application might consider it to be exceedingly complex. However, a software team developing its tenth e-commerce Web site would consider such work run of the mill. A number of quantitative software complexity measures have been proposed . Such measures are applied at the design or code level and are therefore difficult to use during software planning (before a design and code exist). However, other, more subjective assessments of complexity can be established early in the planning process.

Project size is another important factor that can affect the accuracy and efficacy of estimates. As size increases, the interdependency among various elements of the software grows rapidly. Problem decomposition, an important approach to estimating, becomes more difficult because decomposed elements may still be formidable. To paraphrase Murphy's law: "What can go wrong will go wrong”—and if there are more things that can fail, more things will fail.

The degree of structural uncertainty also has an effect on estimation risk. In this context, structure refers to the degree to which requirements have been solidified, the ease with which functions can be compartmentalized, and the hierarchical nature of the information that must be processed.

The availability of historical information has a strong influence on estimation risk. By looking back, we can emulate things that worked and improve areas where problems arose. When comprehensive software metrics are available for past projects, estimates can be made with greater assurance, schedules can be established to avoid past difficulties, and overall risk is reduced.

Risk is measured by the degree of uncertainty in the quantitative estimates established for resources, cost, and schedule. If project scope is poorly understood or project requirements are subject to change, uncertainty and risk become dangerously high. The software planner should demand completeness of function, performance, and interface definitions (contained in a System Specification). The planner, and more important, the customer should recognize that variability in software requirements means instability in cost and schedule.

However, a project manager should not become obsessive about estimation. Modern software engineering approaches (e.g., evolutionary process models) take an iterative view of development. In such approaches, it is possible to revisit the estimate (as more information is known) and revise it when the customer makes changes to requirements.
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