Resource constrained project scheduling: What is my scheduling objective?
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Project scheduling is the act of constructing a timetable for each project activity, and differs in complexity due to the presence of renewable resources with limited availability. In this article, three important aspects of scheduling will be discussed, as given along the following lines:
- Sequencing: scheduling with unlimited resources
- Scheduling: scheduling within limited resource constraints
- Scheduling objectives
A formal definition of project scheduling can be given as follows: It involves the construction of an activity timetable, i.e. the determination of a start and finish time for each project activity, respecting the precedence relations and the limited availability of the renewable resources, while optimizing a predefined scheduling objective.
Sequencing
In the absence of renewable resource constraints, project scheduling boils down to activity sequencing by putting each individual activity as-soon-as-possible in the timetable, respecting the precedence relations, resulting in an earliest start schedule. Consequently, in this scheduling approach, it is implicitly assumed that the minimization of the total project lead time is the scheduling objective.
Two well-known techniques that rely on a straightforward activity sequencing approach are the PERT and CPM scheduling techniques. More information can be found at “The Program Evaluation and Review Technique (PERT): Incorporating activity time variability in a project schedule“ and “The Critical Path Method (CPM): Incorporating activity time/cost trade-offs in a project schedule”.
Scheduling
The presence of renewable resources, constrained by their limited periodic availability, leads to a complexity increase during the construction of a project schedule. Due to the limited availability of resources, the straightforward activity sequencing approach often leads to so-called resource conflicts (see “Linking resources to activities: Resource availability and resource demand“). These conflicts result from over-allocations of renewable resources when activities scheduled in parallel require more resources than available. In order to solve these resource conflicts, activities need to be shifted further in time to periods where resources are still available for the activities. The aim of this scheduling approach is to create a so-called resource-feasible schedule (i.e. a schedule without any resource conflict) and is often a complex task. Moreover, the construction of such a resource-feasible project schedule requires a scheduling objective that needs to be optimized.
Scheduling objectives
A scheduling objective is the objective one aims to reach while constructing a resource-feasible project schedule. Although time is often considered as the dominant scheduling objective, other objectives are often crucial from a practical point-of-view. A non-exhaustive list of possible objectives is given along the following lines:
- Time: minimize the total duration of the project
- Net present value: maximize the discounted cash flows of project activities
- Work continuity: avoid idle time of bottleneck resources in a project
- Leveling: avoid resource jumps but try to balance the use of resources
- Others: Many other scheduling objectives can occur and are often project specific. Moreover, it is logical that in practical environments, a combination of objectives is strived for
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