Thursday, May 5, 2011

Pert Time Calculations

Pert Charts are one of the tools PM's or TD's can use to track projects. While usually used for large and complex projects (For instance Disney could use this system while designing a new ride through implementation and final opening). Pert charts (as does critical path charts) focus on time, not cost as a project driver. Once all activities are entered, with time durations, and predecessors or successors, you can easily see the length of the project by seeing the critical path through the activities.
The critical path will be the path through the path that is the longest duration. There may be a variety of projects that need to be complete prior to project close, but sometimes they do not directly affect project length. There is a lot of information amount PERT charts, CRM, Gantt charts and network diagramming on the web, all of it much more thoroughly written then this so I would encourage you to do additional research.
When doing a PERT, Optimistic time (the minimum time needed) Pessimistic time (Worst case scenario) and Most likely time (best guess) is estimated for each task. What I just learned was how that gets translated into the Expected time duration:
Expected time=(Optimistic Time +4 X Most Likely Time + Pessimistic Time) / 6
Thus, the weighted average of the times creates a realistic duration. Since I usually do all of my estimating in excel workbooks, setting up these PERT calculations would be fairly simple (though perhaps tedious) to implement. It would be an interesting exercise to see if this ultimately leads to an increase in accuracy large enough to account for the additional estimating time.
One can argue that historical data and experience alone lead to better estimates, and to a certain extent that is true, but there are many variables that can affect the actual duration of any task.
Where I currently work we base our hours estimate on skilled labor doing the work. In many cases it is skilled labor that does the work, and the estimates can be fairly accurate. If the shop is full of work, and additional crew is hired, unskilled work (or lower skilled workers) may be put on the job, but their labor rate is also lower – so while the hours calculation will be wrong, the assumption is that the total amount being billed for the hours would work out due to the differences in labor rates. Sometimes this is true, and sometimes it is not. Historically we used to estimate based on who was slated to be assigned to the work. This lead to specific estimates based on the jobs leads capabilities. These estimates could be a little more specific, but were not generalizable, and were more time consuming. And despite being more specific were not any more accurate in the long run. Since we track the hours estimated vs. used, as well as the cost of labor estimated vs. used, it is interesting to see how these numbers can play out.

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