Sections Syndication |
Home : Quantitative Methods : Estimating Duration and Effort
Estimating Duration and EffortEstimating Duration and EffortWe can easily see that the significant metrics in every schedule network are the task durations and the task efforts. These two metrics drive almost all of the calculations, except where paths merge. We address the merge points in subsequent paragraphs. Now as a practical matter, when doing networks for some tasks it is more obvious and easier to apply the estimating ideas discussed in other chapters to the effort and let the duration be dependent on the effort and the number of FTE that can be applied. In other situations just the opposite is true: you have an idea of duration and FTE and the effort simply is derived by applying the equations we described above. Most network software tools allow for setting defaults for effort-driven or duration-driven attributes for the whole project, or these attributes can be set task by task. For a very complex schedule, setting effort-driven or duration-driven attributes task by task can be very tedious indeed. Perhaps the best practical advice that can be given is to select the driver you are most comfortable with, and make selective adjustments on those tasks that are necessary. Consider this idea however: duration estimating ties your network directly to your program milestones. When a duration-driven network is developed, the ending dates or overall length of the network will fall on actual calendar dates. You will be able to see immediately if there is an inherent risk in the project network and the program milestones. Perhaps the most important concept is the danger of using single-point estimates in durations and efforts. The PERT network was the first network system to recognize that the expected value is the best single estimate in the face of uncertainty, and therefore the expected value of the duration should be the number used in network calculations. The BETA distribution was selected for the PERT chart system and the two variables "alpha" and "beta" were picked to form a BETA curve with the asymmetry emphasizing the most pessimistic value. [1] Although the critical path method (CPM) to be discussed below started out using single-point estimates, in point of fact more often than not a three-point estimate is made, sometimes using the BETA curve and sometimes using the Triangular distribution. In effect, using three-point estimates in the CPM network makes such a CPM diagram little different from the PERT diagram. [1]More information on the BETA "alpha" and "beta" parameters is provided in Chapter 2. 378
times read
|
|||
|
|||
