Estimating Durations
Activity durations should be realistic to ensure that forecasted program delivery dates and critical paths are reliable. Activity durations are often mistakenly determined solely by the time available to complete the program. However, activity durations should be based on the effort required to complete the activity, the resources available, and resource efficiency or productivity. This ensures that the dates in the schedule are determined by logic and durations rather than by wishful thinking or estimates that are constructed to meet a particular finish date objective.
If the estimated durations and supporting schedule network logic do not support the target deliverable date, then the program manager and the teams must discuss how to realistically compress the schedule, perhaps by adding more resources, adjusting scope, or setting a later finish date.21
Activity durations should be estimated under most likely or “normal” conditions, not under optimal, “success-oriented” conditions or with padded durations. Most likely conditions for estimated durations imply that duration estimates do not contain padding or margin for risk. Rather, risk margin should be introduced as separate schedule contingency activities to facilitate proper monitoring by management, as discussed in Best Practice 8. Durations also should not be unrealistically short or arbitrarily reduced by management to meet a program challenge.
Activity owners should be responsible for estimating durations for their activities. Activity owners may have experience from similar activities on past projects that can help them estimate the duration of the current activity. If the scheduler creates estimates, it is important that the underlying assumptions and durations are acceptable to the activity owners.
All assumptions related to activity duration estimates should be documented in appropriate detail, such as a record of the methodology used to create the estimate (for example, parametric analysis of historical data or analogy to similar effort) and all supporting historical or analogous data. Documenting the basis for duration facilitates the communication of expectations between activity owners and decision makers and helps in estimating future analogous activity durations.
Finally, activity duration estimates for a WBS element in the schedule should clearly map to and correspond with the basis of the cost estimate for the same WBS element. For example, assumptions for the number of FTE workers underlying the cost estimate for a WBS element should also underlie the duration estimates for the WBS element in the schedule. This mapping need not necessarily be done at the lowest task level. However, at some level of the schedule, duration estimates should be supported by the basis of estimate.
As discussed in Best Practice 1 and Best Practice 3, a comprehensive IMS reflects all the activities of a program yet incorporates different levels of detail, depending on the information available at any point in time. Detailed activities reflect near-term, well-defined effort while less well-defined effort beyond the near term is represented within the schedule as planning packages. Resource assignments and duration estimates vary in detail according to how detailed activities are in the schedule.
Duration estimates for near-term detail activities should be related to the amount of work required, specific resource availability, and resource productivity. Long-term planning packages naturally have less-accurate resource availability and productivity information from which to estimate durations because they can be several months or years long. Estimates for the durations of planning packages are most likely to be based on analogies to historical projects, planners’ experience, or standard productivity rates.
Usually biased estimating has the purpose of achieving an earlier project finish date, often to satisfy management or the customer. That is, the direction of the estimating bias is typically not symmetrical.↩︎