Best Practice 4: Establishing the Duration of All Activities
Duration is the estimated time required to complete an activity—the time between its start and finish. Durations are expressed in business units, such as working days, and are subject to the project calendar. For example, for a standard 40-hour 5-day work week, the duration of an activity that starts on Thursday and ends on Tuesday will be 4 working days, even though it spans 6 calendar days. If the activity is assigned to a 7-day workweek calendar, then the activity will start Thursday and end Sunday. Multiple calendars can be created to accommodate activities with different work schedules.
The definition of duration is different from the definition of work (work is also referred to as effort in some scheduling software). For example, if a painting activity is scheduled for 2 8-hour days and 2 full-time painters are assigned to the job, the duration of the painting activity is 2 days, but the effort associated with painting is 32 hours (that is, 2 8-hour employees for 2 days). Duration is directly related to the assigned resources and estimated amount of required work. Best Practice 3 discusses in detail how duration, resource units, and effort can change in relation to one another.
Durations should be as short as possible to facilitate the objective measurement of accomplished effort. As we discuss in Best Practice 1, the level of detail in the schedule should reflect the information available, the risk inherent in activities, and the intended use of the schedule. In general, estimated detail activity durations for near-term effort should be no longer than the reporting period established by the program. For example, if the reporting period for a construction project is weekly, then near-term activity durations should be one working week or less. If management requires monthly updates, then near-term activity durations should be about 22 working days or less. If activities are longer than the reporting period, activities should have at least one quantitative measurable event within the reporting period. It may be difficult for management to gauge progress on detail activity durations that are too long. Up to a point, the shorter the duration of the detail activity the more precise the measurement of accomplished effort will be. Moreover, shorter durations are needed for areas of work associated with high cost or high risk. Keeping activity durations shorter than the reporting period has additional benefits to tracking progress that we discuss in Best Practice 9.
Long durations should be broken into shorter activities if logical breaks can be identified in the work being performed. If it is not practical to divide the work into smaller activities or insert intermediate milestones, justification for long durations should be given in the schedule basis document. One rule of thumb is to break long activities into enough detail that finish-to-start logic relationships can be identified. Greater activity detail might be necessary if it helps management understand and address the implications of risk and uncertainty.
However, durations that are too short and durations that are too long should be balanced. Very short durations, such as 1 day or less, may imply that the schedule is too detailed and will require more frequent schedule duration and logic updates than necessary. Activities should be decomposed only to the point necessary to identify activity-to-activity hand-offs. Moreover, for a large number of 1- and 2-day duration activities, planners should recognize that people are rarely, if ever, 100 percent productive during an 8-hour day. An actual “pure” productive workday is approximately 60 to 80 percent of an 8-hour workday, because time may be taken up with staff meetings, phone calls, e-mails, and water cooler talks. Also, a long chain of 1-day activities may be assigned to one employee who is assumed to never get sick or take vacation. It is important that activity durations remain realistic. Durations should not be broken up simply to meet an artificial guideline. If the work required for the activity is estimated to extend beyond the reporting period, or if network logic dictates an activity duration longer than the reporting period for some other reason, then the activity duration should reflect this reality.
Certain activities within schedules naturally span more than the number of working days in the reporting period. For summary-level schedules, often created before detailed engineering is complete, durations might be longer than 1 or 2 months and lags might be more common. Within detailed schedules, LOE activities such as management and other oversight activities depend on the duration of the underlying discrete effort, so they span complete phases or even the entire project. LOE activities should be clearly marked in the schedule and should never appear on a critical path.
In some circumstances, it may be beneficial to use long-duration activities in a schedule to reduce complexity. For example, if 30 units of some item need to be constructed and each item has 15 individual steps, the complexity of the schedule can be reduced by creating 30 construction activities rather than 450 step activities. To ensure that long-duration activities can be effectively progressed, they should be monitored using incremental milestones. Incremental milestones—also called inch-stones—are used to track the completion of a long-duration activity. Incremental milestones should represent objective, product-oriented progress on the task and should be managed under a control process. They should enhance the performance visibility of the activity rather than represent arbitrary points in time. Inch-stones used to calculate performance for long-duration activities are sometimes referred to as quantifiable backup data, or QBD.
In addition, planning packages representing summarized or less-defined future work can be several months long. However, the duration of the planning packages must be estimated and they should still be integrated into network logic.
Finally, all activity durations in the schedule should be defined by the same time unit (hours, days, weeks) to facilitate calculating and monitoring the critical path. The day is the preferred time unit.