Delphi

Stop Promising Miracles1
Karl E. Wiegers
Process Impact www.processimpact.com Most software professionals must provide estimates for their work, but few of us are skillful estimators. Many of us haven’t been trained in estimation techniques. We’re too optimistic, with short memories that mask the painful overruns from previous projects. We don’t incorporate contingency buffers to accommodateunexpected events or risks that materialize. And we often overlook necessary aspects of an activity, so that when we eventually confront those tasks, we either perform them—thereby exceeding our estimates—or skip them, perhaps compromising quality in the process. There are several ways to become a better estimator. The most basic approach is to record effort, duration or size estimates as well asyour estimating processes and assumptions, and then record the actual results from each estimated activity. Comparing actual outcomes to the estimates helps generate more accurate estimates in the future. Estimating procedures and templates that itemize tasks help avoid the common problem of overlooking necessary work. Another approach builds on the principle that multiple heads are better than one.Developed at the Rand Corporation in 1948, the Delphi estimation method asks a small team of experts to anonymously generate individual estimates from a problem description and reach consensus on a final set of estimates through iteration. In the early 1970s, Barry Boehm and his Rand colleagues modified this method into Wideband Delphi, which included more estimation team interaction; see Boehm’sSoftware Engineering Economics (Prentice Hall, 1981). Mary Sakry and Neil Potter of The Process Group, a Dallas, Texas-based consulting company, later created a repeatable procedure for performing Wideband Delphi estimation on software projects. Using the Wideband Delphi method provides several advantages over obtaining an estimate from a single individual. First, it helps build a complete tasklist or work breakdown structure for major activities, because each participant will think of tasks. The consensus approach helps eliminate bias in estimates produced by self-proclaimed experts, inexperienced estimators or influential individuals who have hidden agendas or divergent objectives. People are generally more committed to estimates they help produce than to those generated by others. Noparticipant in an estimation activity knows the “right” answer, and creating multiple estimates acknowledges this uncertainty. Finally, users of the Delphi approach recognize the value of iteration on any complex activity.

Wideband Delphi
Wideband Delphi can be used to estimate virtually anything—the number of labor months needed to implement a specific subsystem, the lines of code or numberof classes in an entire product, or the gallons of paint needed to redecorate Bill Gates’ house. I used Wideband Delphi

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This paper was originally published in Software Development, February 2000. It is reprinted (with modifications) with permission from Software Development magazine.

Copyright © 2000 by Karl E. Wiegers

Stop Promising Miracles

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once with a processimprovement group to estimate the effort it would take a particular organization to achieve level two of the Capability Maturity Model. The Delphi method helps you develop a detailed work breakdown structure, which provides the foundation for bottom-up effort and schedule or size estimation. The starting point for a Delphi session could be a specification of the problem being estimated or an initialhigh-level task list or project schedule. The outputs are a detailed project task list; a list of associated quality, process-related and overhead tasks; estimation assumptions; and a set of task and overall project estimates, one from each participant. Figure 1 illustrates the process flow for a Wideband Delphi session. The problem being estimated is defined and the participants selected during...