In my last article, I covered four misconceptions about critical path. Now, I’ll be going through some of the mechanics of finding the critical path in a project, so that you have a better understanding how it evolves. Keep in mind that Microsoft Project’s software performs similar calculations. Manipulation of the network sequence diagram involves simple calculations used to produce the critical path and non-critical paths embedded in the project network plan.
We will assume all activity relationships are Finish-Start (FS) meaning that each predecessor activity has to completely finish before the successor activity can commence. We will also assume that durations are in days. Figure 1.1 shows a visual sample activity-on-arrow (AOA) network. The AOA is a networking diagramming technique in which the activities are represented by arrows and connected at points called nodes to illustrate the sequence of activities. A node is the starting and ending point of an individual activity. The first node represents the start of a project and the last node represents the completion of the project. Path 3, B-D-G-I is the critical path (25 days long) for this small project. Figure 1.2 shows the same representation within Microsoft Project with the red bars displaying the same critical path.
On the start date of your baselined project plan, you will know what your critical path is and the planned completion date. The following are a few examples on how a critical path can get real slippery.
- Sometimes you may have gaps in your critical path due to missing or changing task dependencies, external predecessors, date constraints, or resource leveling. This indicates that your plan is incomplete.
- You may have multiple critical paths (for example, they could be parallel or they could intersect), which is usually more susceptible to having changes and it’s extremely important for PMs to keep their eyes on all of them to avoid a late project completion.
- Most of the time a critical task has zero total slack time, but if it has a negative slack time, it might have been generated by having date constraints or locked-in deadlines. These types of constraints are best to use sparingly. In this scenario, it might be difficult to tell what tasks are really critical, but you can get help by using Project’s Task Inspector (under the Task tab) to view factors affecting the task’s start date. You may also find suggestions on how to fix a troubled task from the Task Inspector.
- If there are two tasks occurring at the same date/time and one is a critical task and the other is a non-critical task, but they both are using the same resource, the PM needs to allocate the resource to the critical task first. Then, the PM would use the slack time on the non-critical task to delay the start of this activity. Going through this exercise prevents the critical path from changing and extending the project’s completion date.
There are many ways to look at and monitor the status of a project plan’s critical path to see if it’s becoming slippery or changing. For example, showing critical tasks in the Gantt chart view as shown in Figure 1.2.
- The left pane shows the columns for field values, and the right pane shows the Gantt chart timescale. If you right-click the left side pane, you can insert the “critical” column to find out if the tasks are critical or not. Furthermore, if you right-click the left pane, you can insert the “Total Slack” column, and if you see zero, you’ll know it’s a critical task. Otherwise, the project doesn’t consider it a critical task.
- Click the View tab, and then select the “critical” filter. The critical task bars appear in red (similar to Figure 1.2).
- Click the Format tab, and then select the Critical Tasks check box. The critical task bars appear in red.
- Click the Report tab, and then In Progress reports. You can select Critical Tasks to gets its report, which includes the name, start, finish, % complete, remaining work, and resource fields.
- Click the Report tab, and then in Visual Reports. Then select Microsoft Visio’s Critical Tasks Status Report. From here, the selected data from project is exported to Visio, and is displayed as a hierarchical thumbnail graph that shows the percentage completed for each critical task. You can rearrange, format the report, and save it as a Visio file.
Understanding both the critical path and slack concepts is vital for effective time management and requires that the PM keep these parameter values in mind as the project unfolds. Understanding task slack (or float) gives the PM flexibility in scheduling the start of a particular activity, which helps to facilitate prioritization of resource allocations across the project plan. If you want to shorten a project’s duration, always concentrate on what can be done to the activities that make up the critical path, but keep in mind this usually is not a trivial exercise. Typically 20% or fewer of all the project’s activities are ever on the critical path. Also, when you shorten a project’s duration, you must remember the project’s triple constraints – time, cost, and scope. If one of the constraints has to change, it will alter one or both of the other constraint(s). For example, if you have to decrease the duration (time), you might need to increase budget (cost), because you have to hire more resources to do the same work in less time, or you might have to reduce the scope of the project.
Always double-check your project plan. Mistakes, like missing tasks and/or duplicate tasks, can linger affecting your critical path. For example, task dependencies that should not be there, tasks with no resources assigned, summary tasks that have resources assigned, over-allocated resources, and duration values that seem too high or low. Make sure you have enough milestones (important events or accomplishments) in your plan for project status reporting and the durations to equal zero. Milestones are like traffic signs, a signal that you have reached an important point or decision in a project. I have seen many project plans that have no milestones, have only one milestone of “end of project”, have milestones without zero durations or assigned resources. This is always a red flag that this project plan is not well thought-out! Don’t let such happen to you!