Resource Leveling
Resource leveling adjusts the scheduled start of activities or the work assignments of resources to account for their availability. Primarily the organization that has control of the resources uses it to smooth spikes and troughs in resource demands created by the sequencing of activities in the schedule network. Frequent peaks and troughs in resource requirements indicate their inefficient use inasmuch as frequent mobilization and demobilization of resources is disruptive. For example, a schedule that requires 30 software engineers one week, none the next week, and 25 the next is disruptive and inefficient. A more manageable resource plan should be sought.
Leveling can be as simple as reassigning work from overallocated resources to underallocated resources or delaying the start date of activities until the required resources are available. Leveling may also develop into a complex trade-off between the required duration of the plan and the availability of myriad resources. Resources can be leveled automatically with scheduling software or manually by managers and planners or both. Whatever the method, the goal of resource leveling is to finish the project on time or early, if possible, with the resources realistically expected to be available throughout the entire plan. Leveling resources allows management to identify “critical resources”—that is, resources that will delay the project finish date if they are not available for specific activities. Resource leveling is ideally integrated into scheduling and updating to ensure the best possible trade-offs between resources and time and to ensure that the schedule remains reliable throughout its life.
Typically, activities delayed by resource leveling have the greatest free float available and the fewest assigned resources.19 Leveling resources by shifting activities with free float minimizes the effect of the delayed activities on the project as a whole and minimizes the number of resources that must be reassigned work. Given the amount of float available in the schedule and the original assignment of resources, resource leveling can have little to no effect on a schedule, or it can severely delay the forecasted end date of the project. Resources should be leveled only to reallocate resources to reduce spikes and troughs by absorbing available float.
In other words, resource leveling should not delay a completion date if possible. If the outcome of resource leveling is a delayed completion date, then resources are an inevitable constraint, and an alternative approach to the project plan is needed. If resources are assigned to activities realistically according to the sequence of planned activities and no resources are overallocated, then the resource-leveled scheduled will be the same as the original CPM schedule.
An example of resource leveling is given in figures 26 and 27. Figure 26 shows a selection of the sequence of events for selecting subcontractors for house construction, as scheduled according only to predecessor-successor logic. Once the owners select a general contractor, they can confer with the general contractor to select the subcontractors: surveyor, excavator, concrete supplier, plumber, electrician, and the like. Once an individual subcontractor is selected, the owners and general contractor can select and approve the necessary material. For instance, after selecting a plumber, the owners and general contractor can select the plumbing fixtures. These activities are planned to occur while the owners wait for the general construction permit to be granted, which is expected to take 20 days. From a network perspective, all subcontractors can be selected at the same time once the owner selects the general contractor and, once each subcontractor is selected, the respective materials can be selected. This sequence of activities is portrayed in figure 26.
Figure 26: Resource Overallocation in a Correctly Sequenced Network
However, once resources are assigned to activities, it quickly becomes apparent that the plan is not feasible. Figure 26 gives the hours necessary for two owners and one general contractor to complete the selections for each day. The resource leveling of these activities is straightforward, because the owners and the general contractor have time available to select subcontractors while they wait for approval of the general construction permit. In fact, the “subcontractors selected and equipment and materials approved” milestone has over 20 days of free float because the selection activities are occurring parallel to the wait time allotted for construction permit approval. This allows management to spread the selection of subcontractors over the next several weeks without affecting activities succeeding the subcontractor selection milestone. Figure 27 shows the results of manual resource leveling for these activities.
Figure 27: Resource Leveling in a Correctly Sequenced Network
The activities are now spread over 2 weeks, leaving the owners and general contractor busy but not overallocated. This effort profile also frees the general contractor to work with other clients. Finally, the subcontractor selection activities are leveled within the available free float, which does not affect either activities beyond the subcontractor selection milestone or the original critical path.
Because changes to the schedule from limited resource availability can alter float and critical path calculations, it is important that changes to resolve the resource conflicts be thoroughly documented and that everyone understand them.20 Planners and management should always carefully examine output from automatic resource-leveling routines and it should be tempered or adjusted where necessary. Automatic leveling may prove inefficient, as when it delays activities when resources are only partially available and, thus, prevents activities while the project awaits the full complement of resources.
It is important to note that decisions made from incorrect data assumptions will themselves be incorrect. This is especially true if a CPM schedule is being overridden by resource-leveling decisions based on summary-level or incorrect resource assumptions. Resources should be leveled only in detailed schedules that include detailed resource estimates supported by historical data and sound estimating methodologies. Without specific resource assignments, effects and costs cannot be accurately estimated and tracked. If a schedule is resource leveled, it is important to check the date of the last leveling against the date of the last changes to resources and resource availability. This will ensure that actual resource assignments have been aligned with the proposed leveling in the model.
Resource leveling should never be applied to summary schedules or when resources are specified at such a summary level that the concept of availability cannot be applied. Incorrect resource assumptions (usually in the form of unwarranted optimism) lend unreasonable credence to a resource-leveled schedule, and the resulting leveled schedule will convey a false sense of precision and confidence to senior decision makers.
Free float is the time in which an activity can be delayed without affecting any successor (see Best Practice 7).↩︎
Logic specifically employed to solve resource leveling issues once the plan is baselined is known as “preferential logic” or “soft logic.” Preferential logic dictates a desired sequence of activities that is not entirely necessary. That is, the logic is not related to the work itself but reflects management’s plan for realistically executing the work. Conversely, “engineering logic,” or “hard logic,” dictates an order of activities that must occur regardless of preference. For example, concrete curing must always occur after pouring concrete. Using preferential logic to address resource leveling may be subject to the program’s schedule change control.↩︎