With every project, at least one stream of tasks has no slack between any of the sequenced tasks. Each task, except the first, depends upon completion of a predecessor before it can commence. Many tasks have more than one predecessor. The last completed predecessor determines the successor task's start date.
Predecessors that finish earlier leave some slack time before the critical task successor begins. Therefore, if non-critical tasks finish late but do not utilize all their slack time, the project timeline is not affected. Also, if there is plenty of slack (that is, slack is greater than the possible time over run of the non critical path task), these tasks do not have to start as soon as possible. It is important, however, that they finish well before the critical path task is scheduled to start.
A project manager must take care not to delay a non-critical task start for so long that no slack remains. Where there is a cushion, keep some cushion. Otherwise, this task will have to be defined as a critical path task and included in risk analysis. Some discretionary flexibility is always desirable. However, if the project effort is proceeding and the task leader and the project manager determine that a start delay for one task must be allowed that uses up all the slack, downstream task leaders must be warned of any increased risk.
Gantt chart software can identify critical path tasks. Microsoft Project 2002® can depict them with bars that are shaded or colored differently from other tasks (see Figure 7-2A and 7-2B). It also can print critical path task names in a different color or typeface, when instructed to do so. One Microsoft Project 2002® special report lists critical path tasks and their characteristics. This report is available through computer commands.
When a critical path is displayed on the Gantt chart, the line connecting a non-critical path task to its successor will extend horizontally until the critical path task start date is reached (as shown in Figure 7-1). This line represents slack. Slack lines do not stand out on a large complex project. This is particularly true when a series of connected tasks end with the last task showing slack before a critical path task can start.
The critical path arrow diagram is a tool that we find very useful for monitoring and managing resources. It is currently not found in many project management software packages. Preparing it longhand is easy, once the Gantt chart is finished. It is a management tool that is intended for project managers. You can, of course, create one for your own use.
The critical path is essential to IPM because risk analysis is performed for all tasks on and closely related to the critical path tasks. Before risk analysis is undertaken, all team members must know the critical path task names.
Using graph paper, the project manager (or an assistant) can construct longhand an arrow diagram critical path display. The technique is simple. The necessary information can be found in the Gantt chart. (Although software can display the critical path on the Gantt chart [refer to Figure 7-2], the arrow diagram works better for managing larger complex projects because slack may be hard to find on a Gantt chart visually and because the entire project plan cannot be displayed on one screen for very large projects.)
The project manager creates the critical path arrow diagram as soon as the Gantt chart is completed. The chart should be printed and distributed to every team member before the risk analysis discussion meeting. Remind each task leader that they will need to give special attention to the critical path arrow diagram for risk analysis.