As every project progresses through it’s lifecycle, the team’s forecast will evolve. The forecast value may move up or down, however, the accuracy of the forecast should always increase. The basis for increasing accuracy is that all estimates are forecasts with some level of uncertainty and as the project progresses the unknowns/uncertainty will decrease. This holds for forecasting any of duration, work effort, or cost.
There are two important concepts in the below figure:
1) We see the team’s forecast (solid middle line) moves up and down as time progresses; and,
2) the range in value between the High and Low Estimate decreases in steps at each phase.
A key action for the Project Manager is to communicate to all stakeholders that early estimates have higher uncertainty. As part of communication with management and finance stakeholders, I usually ask for a reserve to be added onto my estimates based on the higher uncertainty of estimates and potential negative impact of risks. This amount can be progressively reduced and “given back” as the project progresses over time.
Some types of projects inherently have high uncertainty during initiation and planning. For example, integration of custom software. When faced with projects involving high level of unknown, the Project Manger should use “Three-Point Estimating”. This technique will include the full range of possible values of the estimate and reduces bias that can lead to a highly optimistic or pessimistic estimate.
I usually create custom fields within Microsoft Project 2010 to capture and calculate the three point estimates. The approach is also called PERT. The formula is PERT Estimate = (Optimistic Estimate + 4 X Most Likely Estimate + Pessimistic Estimate) / 6.
Other project teams that work on a high number of similar projects will develop good enterprise knowledge for making estimates. An example would be an energy and gas company that knows 2 resources can lay pipe at 20 metres per hour and the material cost is $150 dollars per foot. Estimates in these situations can be very accurate, from an early stage.
A Project Manager may have little control of the level of uncertainty or risk when handed a new assignment. However, appropriate application of the concepts above will lead to successfully managing and quantifying estimates of duration, effort, and cost.