We continue with our series on task definition and duration estimation. In the previous post we focused on decomposition of the work breakdown structure to define tasks. This time, we describe the process for estimating task duration.
Task Duration Estimation
Informally, to estimate task/activity duration, list the tasks to be performed to accomplish the project or super task. Decompose complicated tasks into subtasks. Collect time estimates for the tasks from the list of project participants and stakeholders. Consider the time estimates from similar tasks and/or projects. Evaluate each estimate then for the chosen add buffer time, 25-50% to each, for unforeseen events—foresight must consider oversight. Make a schedule of the itemized tasks. Evaluate the resulting duration for the task list or project.
Formal Estimation Steps
Formally, task/activity duration estimation, the Activity Duration Estimation process, is part of project schedule management. One of several inputs to the process is the activity list to which estimation tools and techniques are applied to produce duration estimates. These tools and techniques are expert judgment, analogous estimating, parametric estimating, three-point estimating, bottom-up estimating, Data analysis (alternative analysis, reserve analysis,) decision-making, and meeting. Here, we highlight, author’s preference, expert judgment, analogous estimating, parametric estimating, three-point estimating, and bottom-up estimating:
- Activity time estimate gained from persons or groups with specialized training is known as expert judgment
- In analogous estimating historical time data from similar activities and projects are used, directly, to estimate the current task, group of tasks, or project duration. It is considered a less accurate method than other estimation techniques
- Parametric estimating, which can be more accurate than other methods, employs statistical relationships tying historical project data and other variables to determine task/activity and project duration. For example, duration can be calculated by multiplying time per unit by the number of units.
- The three-point estimation technique, utilized when historical data are scarce, improves a single-point duration estimate by incorporating uncertainty and risk in the estimation. First, a “most likely” (tM) estimate is made based on available resources, dependencies on other participants, and potential stoppages for an activity. Then, an optimistic (tO) duration estimate is based on scrutiny of the best-case scenario for an activity. Thirdly, a pessimistic (tP) duration estimate is determined based on a worst-case scenario. Finally, an expected duration estimate (tE) is calculated from the three prior estimates using the formula: tE = (tO + tM + tP) / 3
- Bottom-up estimating is used when an activity’s duration estimate is not trustworthy. The activity is further decomposed and duration estimates are made for its elements. These estimates are then summarized to give the top activity duration estimate.
In Conclusion
Task definition and duration estimation are crucial to project success. The practitioner must devote his time and effort to using the various tools and techniques to accomplish them. The urgency for task definition in projects differ from that involving ongoing business process due to projects un-forgiveness for delays and cost. At the very least consider the informal process for task definition if the formal process is too convoluted. In task duration estimate consider the informal process that does not involve calculus-like calculations if the implication for overall project duration and budgets will not be severely impacted. On the other hand, if you have a large project or are fearful of going astray use the formal process for duration estimates.
–Richard Thomas
