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 estimation assumptions.
The estimators have reached consensus on how their individual estimates were synthesized into a single set with an acceptable range.
Now you must decide what to do with the data. You could simply average the final estimates to come up with a single-point estimate, which is what the person who requested the estimate probably expects to hear. However, a simple average is likely to be too low, and there is merit in retaining the estimate range. Estimates are predictions of the future and the range reflects the inherent fuzziness of gazing into the crystal ball. Following are several ways to present the results from the Delphi session, using the five estimates from Round 3 in Figure 11-5 as an example.
Present just the average of the final individual estimates. (example: 1974 hours)
Present a single estimate calculated as follows (example: 1966 hours):
Present a range, with the average of the final estimates as the planned case, the minimum value as the best case, and the maximum value as the worst case. (example: 1974 hours planned, 1650 hours best case, 2250 hours worst case)
Present a range, with the average as the planned case, the upper bound being (maximum value – average) and the lower bound being (average – minimum value). (example: 1974 hours, +276 hours, –324 hours)
Each estimate has a certain probability of coming true, so a set of estimates forms a probability distribution. (See Chapter 10.) Watts Humphrey described a mathematically precise way to combine multiple estimates and their uncertainties to generate an overall estimate with upper and lower prediction intervals (Humphrey 1995). Another sophisticated approach is to perform a Monte Carlo simulation to generate a probability distribution of possible estimate outcomes based on the final estimate values (Campanis 1997).
The results of a Wideband Delphi session might not be what the movers and shakers want to hear. But this information helps them determine whether they want to plan their project at a 10 percent confidence level, a 90 percent confidence level, or somewhere in-between. Be sure to compare the actual project results to your estimates to improve your future estimating accuracy.