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A resource is overallocated when it is assigned to work more hours during a given time period than it has available for work on the project. The number of hours the resource has available for the project during any given time period is determined by two values that you define:
Multiplying the units available for the resource by the calendar working hours in a given time period determines the hours of work the resource has available for that time period. Table 11.1 shows several examples of the work availability calculation. A single work resource typically has 8 hours available per day and has maximum available units of 100% (that is, one full-time unit). Remember that a resources calendar simply defines availability to work and not necessarily what the actual work output will be.
Case A shows that this resource can be assigned up to 8 hours of work each day. But, if the employee has only 4 hours of working time per day defined on his calendar, Case B shows that you are limited to assigning no more than 4 hours per day to that resource. Case C shows a consolidated or group resource with 300% units available and 8 hours of working time per day defined on its calendar. This resource can deliver up to 24 hours of work in the time period.
Table 11.1. Determining the Working Hours Available for a Resource
Overallocations generally occur for two reasons:
When Microsoft Project calculates a task schedule for you, it can easily create an overallocation by scheduling multiple-task assignments for a resource during the same time period. Before resource assignments are made, Project schedules tasks in forward-scheduled projects to start as soon as possible, based on three factors: the first possible date (as determined by the start of the project and any predecessors for the task), the earliest date that leaves constraints satisfied, and the next available working time on the project base calendar (or task calendar, if one is assigned).
Forward-scheduled projects are those that are scheduled from a fixed start date . Project calculates the finish date of the project. This explanation and most others in this chapter are worded in terms of forward-scheduled projects. Microsoft Project automatically makes tasks in such projects As Soon As Possible tasks, to minimize the project duration and achieve the earliest finish date for the project.
If a project is scheduled from a fixed finish date , Project automatically makes the project's tasks As Late As Possible tasks, in order to minimize the overall duration for the project and achieve the latest possible project start date.
When you assign a resource to work on a task, Project substitutes the resource calendar for the project base calendar and schedules the task assignment on the first available date on the resource calendar that meets the conditions described in the preceding section. However, Project does not normally look to see whether the resource is already assigned to other tasks during the times it schedules for the new assignment. Because Project ignores existing assignments when scheduling new assignments, it is very possible for a resource to be overallocated.
If there is an assigned task calendar, the dates that are scheduled must normally be working dates on both the task calendar and the resource calendar. If the task field labeled Scheduling Ignores Resource Calendars is checked, the resource calendar is ignored and the task calendar is the only calendar considered . If this results in work being scheduled during periods that are nonworking times on the resource calendar, Project does not consider this to be an overallocation of the resource.
The default behavior for Project is to ignore other assignments. However, you can change the default behavior and have Project check for other assignments each time it schedules a resource and, if necessary, delay the new assignment until the resource is free to work on it. Before you decide to change this default, however, you should read the rest of this chapter, especially the section "Understanding the Pitfalls of Automatic Leveling."
If there is more than one assignment for a resource during a given time period, the combined work and units for that period might exceed the resource availability, as illustrated in Table 11.2 for Scott Adams. The first two rows in the table spell out the resource availability: Scott has 100% units available for assignments and 8 hours of working time on each day; thus, his available work is 8 hours each day.
Scott has three assignments during this week, and on Thursday two of them overlap. Task C starts before Task B is completed. The total assigned units on Thursday is 150% (which exceeds Scott's max units), and the total assigned work on that day is 12 hours (which exceeds the 8 available hours on the calendar). Therefore, Scott is overallocated on Thursday.
Table 11.2. Scott Adams's Assignments for One Week
Note that this example involves only 36 hours of work for the week, which is less than the 40 hours of work available for the week; therefore, averaged out over the week, Scott Adams is not overallocated. In fact, if you could just reschedule the 4 hours of work assigned for Thursday to take place on Friday, there would be no overallocation.
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