Scheduling is generally one of the critical variables to project success for engineers. In spite of this, most engineers don’t understand the basics of project scheduling, except for remembering it from their college days. In this article I will document the two primary scheduling techniques available to the engineering project manager.
Gantt charts do an excellent job of showing the time required for various tasks and work best for an “overall picture” of where the project is at in relation to all the varied tasks. At its simplest, its just a bar chart with the various tasks itemized.
Gantt charts can also contain dependencies, such as:
- Finish to Start: Task B can start after Task A has finished.
- Start to Start: Task B can start once Task A has started.
- Start to Finish: Task B must finish before Task A starts.
- Finish to Finish: Task B must finish before Task A finishes.
- Percent Complete: Task B can only proceed through X% complete before Task A is Y% complete
These dependencies can be shown on the Gantt chart as follows:
PERT stands for Program Evaluation and Review Technique. This method is better for managing complex dependencies but does not provide as good a “snapshot” of the overall project timeline as Gantt charts do. Activities are shown as a series of bubbles and arrows:
Dependencies are obvious; Task 1 needs to be complete before task 2 starts. As you can probably imagine by now, a complicated network is much easier to manage with this method.
The Critical Path Method
On order to determine the effect of changes on the network, we analyze it with the critical path method. Since there exists only one longest path through the network, tasks on this path have a direct impact on the overall project completion date. This path is essential for resource scheduling and allocation and it is called the critical path.
The tasks that are not on the critical path have some slack time, also called float time.
To perform the analysis, we use the following steps:
- Starting with the first task, determine the earliest possible start date (TE) for each task. This is called the forward pass.
- Starting with the last task and working backwards, determine the latest possible finish date (TL) for each task without affecting the project completion date. This is called the backward pass.
- Highlight the critical path. For each task, Slack Time = TL – TE. Where Slack Time = 0, the task is on the critical path.