In the final analysis, the art of estimation isn't very well equipped to take the last step in producing an accurate schedule. Schedules vary too much based on too many factors. The easiest and most accurate way to compute a schedule is to use a tool such as Construx Estimate. [1]
What happens when we use Construx Estimate to compute a schedule estimate for the case study? If we calibrate Construx Estimate with the historical effort and schedule data that was presented in Chapter 19, Construx Estimate calculates a schedule of 12.2 calendar months, with a 20% to 80% likely range of 11.6 to 12.9 months. With industry-average data, the nominal is 15.8 months and the range is 13.2 to 21.5 months!
The discrepancy between the nominals and ranges computed using historical data and industry-average data once again illustrates the benefits of using historical data.
[1]Construx Estimate can be downloaded for free from www.construx.com/estimate.