Project Scheduling

The WBS effectively shows hierarchical relationships among tasks but it does not show the sequence in which tasks or work packages need to be performed. Preparation of a realistic project schedule depends on understanding the workflow and placing the tasks in the proper sequence. Many project tasks can be performed concurrently but certain tasks must be completed before other tasks can start.  This is known as the sequence constraints. Every organization has to work with limited resources but resource constraints should not be a factor in determining task relationships. Many tasks are interdependent and have to be completed in a logical order regardless of availability of resources. The figure below illustrates a few examples of common task relationships.


Common Task Relationships


Network diagrams are commonly used to display the logical sequence and relationships among the tasks as well as identify project milestones that signify important events in the project such as completing the major project deliverables. Network diagrams are typically created for larger projects using commercially available project management software programs. The simple network diagram shown below illustrates how the project flows from start to finish and what types of dependencies exist between the tasks. For example, task B is dependent on the completion of task A before it can get started. In other words, task A is a predecessor of task B.


Network Diagram


In order to estimate duration of the entire project it is necessary to determine the type, quantity and availability of resources required to complete each individual task. The resources typically considered in estimating activities include human resources, equipment, technology, and materials. Resource estimates are important not only to estimate duration of tasks but also to develop project cost estimates. Availability of resources and the amount of time needed to perform the task are often related because certain tasks can be completed much faster by increasing the assignment of resources.  However, adding more resources does not always result in increased productivity and reduced duration of the task. This is particularly true in the case of research, innovation and product development projects that involve scientists and other knowledge workers. It should be also pointed out that the task effort and task duration may or may not be the same. For example, let’s assume that it takes the total of 8 hours or one day of effort to develop a new MRI screening form. If the project team member assigned to perform this task can finish it within the same day then the task effort and task duration are the same. However, if the project team member can afford to work only 2 hours per day on this task then duration of the task will be 4 days even though the effort required to complete the task is still 8 hours.

Accurate task duration estimates are essential for developing a realistic project schedule. To improve the accuracy of estimates the project manager should consult with the team members who will actually perform the work, seek advice from experienced project managers who have managed similar projects, review historical information for similar tasks performed in previous projects, and involve key project stakeholders in the process. Additional approach to improve the accuracy of estimates is to use a technique known as three-point estimates. The three-point estimates include the optimistic estimate based on a best-case scenario when everything goes perfectly well, the pessimistic estimate based on a worst-case scenario when major problems are encountered, and the most likely estimate based on availability of resources and assumption that some relatively minor problems will crop up. An overall task duration estimate is obtained by calculating average of the three estimates.

Since realistic schedule is the critical output of the project planning process the project manager should carefully consider task dependencies, availability and allocation of resources and document all the scheduling assumptions for the project. The schedule will likely need to be adjusted and revised several times before creating the final project schedule. To account for the unexpected problems and schedule risks associated with some tasks the project schedule may include some contingency time or buffer but not more than 10% of the initial task duration estimate. Since developing the schedule manually for large and more complex projects can be a very time-consuming process and may involve many tedious calculations, the use of project management software is recommended. Once the schedule is approved by the project sponsor it is used as the baseline for tracking and monitoring purposes.