Wideband Delphi
The Wideband Delphi estimation method is a consensus-based estimation technique for estimating effort. The Wideband Delphi estimation process is especially useful to a projectmanager because it produces several important elements of the project plan. The most important product is the set of estimates upon which the project schedule is built. In addition, the project team creates a work breakdown structure (WBS), which is a critical element of the plan.
Description
It is possible to improve the accuracy of an estimate by combining the judgement and estimates made by a number of people. Boehm (1981) describes the Wideband Delphi technique for combining the opinions of experts to make a size estimate. Each expert is provided with a specification and an estimation form.
Wideband Delphi works because it requires the entire team to correct one another in a way that helps avoid errors and poor estimation. While software estimation is certainly a skill that improves with experience, the most common problem with estimates is simply that the person making the estimate does not fully understand what it is that he is estimating. He may be an experienced software engineer, but if he has not fully explored all of the assumptions behind the estimate, the estimate will be incorrect. Delphi addresses this problem through the discussion of assumptions and the generation of consensus among the estimation team members.
The Delphi method helps you develop a detailed work breakdown structure, which provides the foundation for bottom-up effort and schedule or size estimation. To generate a sound estimate, a project manager must have:
- A work breakdown structure (WBS), or a list of tasks which, if completed, will produce the final product
- item An effort estimate for each task
- item A list of assumptions which were necessary for making the estimate
- item Consensus among the project team that the estimate is accurate
Team members make assumptions about the work to be done in order to deal with incomplete information:
- Any time an estimate must be based on a decision that has not yet been made, team members can assume the answer for the sake of the estimate
- Assumptions must be written down so that if they prove to be incorrect and cause the estimate to be
inaccurate, everyone understands what happened - Assumptions bring the team together very early on in the project so they can make progress on important decisions that will affect development
How To
The Wideband Delphi approach can be executed as follows: \cite{McConnel_BlackArt05}
- The Delphi coordinator presents each estimator with the specification and an estimation form
- Estimators prepare initial estimates individually. (Optionally, this step can be preformed after step 3.)
- The coordinator calls a group meeting in which the estimators discuss estimation issues related to the project at hand. If the group agrees on a single estimate without much discussion, the coordinator assigns someone to play devil's advocate.
- Estimators give their individual estimates to the coordinator anonymously.
- The coordinator prepares a summary of the estimates on an iteration form and presents the iteration form to the estimators so that they can see how their estimates compare with other estimators' estimates.
- The coordinator has estimators meet to discuss variations in their estimates.
- Estimators vote anonymously on whether they want to accept the average estimate. If any of the estimators votes "no," they return to step 3.
- The final estimate is the single-point estimate stemming from the Delphi exercise. Or, the final estimate is the range created through the Delphi discussion and the single-point Delphi estimate is the expected case.
The coordinator should take care to prevent people with dominant personalities from unduly influencing the estimate. Software developers aren't known for their assertive personalities, and the most reserved person will sometimes have the best insights into the work being estimated.
Example
Let’s assume that you wish to estimate the total amount of work effort (typically expressed in labor hours) needed to complete a certain project. The estimation process begins with each participant independently developing an initial list of the tasks that will have to be completed to reach the stated project goal. Each participant then estimates the effort each task will consume. Break each activity down into tasksthat are small enough to estimate accurately.
Since radically different assumptions can lead to wide estimate variations, record any assumptions you made while preparing your estimates. For example, if you assumed that you will purchase a specific component library or reuse one from a previous project, write that down.
Another estimator might assume that the project will develop that library, which will lead to a mismatch between your two overall estimates.The moderator begins the estimation meeting by collecting the participants’ individual estimates and creating a chart such as Figure \ref{fig:WidebandDelphiRound1}. Each estimator can see where his or her initial value fits along the spectrum. The moderator does not identify who created each estimate; this anonymity is an important aspect of the Delphi technique. Anonymity prevents an outspoken colleague from intimidating the other participants into seeing things his or her way. Each estimator reads his or her initial task list, identifying any assumptions made and raising any questions or issues, without revealing which estimate was theirs. Each participant will have listed different tasks that need to be performed. Combining these individual task lists leads to a more complete list than any single estimator is likely to produce. After this initial discussion, all participants modify their estimates concurrently. All estimators can add new tasks to their forms and note any changes they wish to make to their initial task estimates.
The cycle of revising the task list, discussing issues and assumptions and preparing new estimatescontinues until:
- you have completed four rounds
- the estimates have converged to an acceptably narrow range (defined in advance)
- the allotted estimation meeting time (typically two hours) is over
- all participants are unwilling to alter their latest estimates
The estimation process is completed when specified exit criteria are satisfied. Exit criteria help you determine when a process execution is done, so you can declare victory and move on with your life. Typical Wideband Delphi exit criteria are that:
- The overall task list has been assembled
- You have a summarized list of estimating assumptions
- The estimators have reached consensus on how their individual estimates were synthesized
into a single set with an acceptable range.