Calendars
Calendars in schedules specify valid working times for resources and activities. Resources can be assigned to calendars to define their availability. The availability of a resource in turn affects the dates and elapsed duration of the activity to which it is assigned. Activities should be directly tied to task calendars, which will define the valid times an activity can be worked. Calendars are defined by the number of work days per work week as well as the number of hours available each work day. Special nonwork days, known as exceptions, are defined in resource and activity calendars. Holidays and plant shutdown periods are examples of exceptions at the activity calendar level. Resource managers are responsible for identifying and assigning the correct calendar to their resources.
As is described in Best Practice 2, the proper use of resource and task calendars usually precludes the need for soft constraints in schedules. For example, a SNET constraint may be used to prevent a testing activity from beginning too soon while the testing facility is in use for an unrelated project. Instead of a SNET constraint, the test facility should be defined as a resource within the schedule and assigned to a calendar whose exceptions represent days other projects are using the facility. If the testing facility becomes available sooner or later than originally planned, the scheduler need only update the exceptions within the testing facility resource calendar. Task calendars may also be employed to represent a suspension of effort. For example, if severe weather requires the suspension of pouring concrete, the activity can be assigned to a specific calendar that prevents the work from being performed on certain days.
Another use for a resource calendar might be to exclude seasonal days from work. Outdoor construction probably cannot be conducted when the ground is frozen or rain is intense. Such an activity could start before the rain or the freeze but would then have to continue after the end of the exception. A calendar that excludes November through February for bad weather could be assigned to all outdoor construction activities. Such dates are well known from many years of experience. Defining resource calendars in this way allows for properly scheduling activities automatically according to network logic. Automatically updating the schedule gives greater confidence in float calculations and the derived critical paths. Resource calendars must also be adjusted for planned overtime—to allow a resource to work in an otherwise nonworking period.
While resource calendars allow for greater insight into resource availability, having too many calendars may interfere with critical path analysis because calendars can affect float calculations. The benefit of using resource calendars to track exceptions for individual resources should be tempered with their possibly negative effects on the critical path (see Best Practice 6 for more information). In addition, the administration necessary to build and maintain many resource calendars may quickly outweigh their benefits.
Program managers must ensure that calendars are properly defined because schedules can incorrectly represent the forecasted start, finish, and durations of planned work if resources are assigned an incorrect calendar. A common mistake allows all activities within a schedule to simply adhere to the default calendar within the scheduling software. However, a default calendar rarely has national holidays appropriately defined as exceptions and does not define specific blackout periods or related exceptions. Similarly, the general project calendar that would have excluded holidays still may not represent the work practices of all resources. For instance, a testing facility may work 24 hours a day while some personnel work 4 10-hour days a week.
Additionally, if management has planned that work will be performed 7 days a week within the schedule, it is crucial that the people assigned activities are aware of the schedule. Planning effort can prevent unexpected delays in the project and the unnecessary use of schedule contingency. Establishing realistic calendars provides for greater accuracy of dates and may reveal opportunities to advance the work. Case study 9 shows how incorrect calendars can affect planned dates.
The pilot schedule for the Department of Homeland Security’s Transportation Worker Identification Credential (TWIC) program included duration estimates for all activities, but GAO could not be certain of their reliability. Nearly 86 percent (259 of 302) of the activities identified in the schedule were assigned to a 7-day calendar that did not account for holidays. While pilot sites may normally operate on a 7-day schedule, resources for conducting pilot activities such as installing readers and associated infrastructure such as cables and computers or analyzing the results of pilot data may not be available on weekends. By using a 7-day calendar, the schedule inaccurately indicated that approximately 28 percent more workdays were available each year than actually were available. In addition, our analysis of an earlier TWIC schedule found that calendars did not include the appropriate exceptions for holidays. For example, multiple pilot sites were scheduled to finish submitting pilot test data on Christmas Day.
Best practices in project management include obtaining stakeholders’ agreement with project plans, such as the schedule. Because the schedule was not shared with the individual pilot sites, responsible pilot officials had no opportunity to comment on whether the 7-day schedule matched available resources. Therefore, pilot participants may not have the resources, such as employees who can work on weekends, to meet pilot goals.
If an activity is defined as taking 60 days, or approximately 2 months using a 7-day calendar, the reality may be that participants work a 5-day workweek with the result that the activity takes approximately 3 months to complete—1 month longer than scheduled.A best practice when considering the duration of activities around the U.S. holiday season (Thanksgiving through New Year’s Day) is to recognize that productivity is generally low then and that many workers take extended holidays during this time.22
Schedule risk analysis can handle this phenomenon with probabilistic calendars, where any day has less than a 100 percent chance of being a working day, with that percentage reflecting the experience of people taking time off during a holiday season.↩︎