The critical path is the longest path (by time) through the project. It is the path with the least Total Slack.

The critical path is made up of the tasks that are the most likely to cause us to miss our project end date.

It is NOT, necessarily, the path with Total Slack = 0. If we use a Must Finish On constraint on our last project milestone (Target Complete) as recommended in the previous section, the Total Slack on the critical path may be positive, negative or zero.

Remember, that if the Total Slack on our critical path is:

  • Positive – The Total Slack indicates how many days early our project is currently forecast to finish.
  • Negative – The Total Slack indicates how many days late our project is currently forecast to finish.
  • Zero – It means that no task in the path can slip or extend even one day without affecting the project end date.

As was illustrated in the previous section Constraints and Deadlines, if we use:

  • No finish date constraint, the Total Slack on our critical path is always 0.
  • A Deadline date to assign our due date, the Total Slack on our critical path will be 0 or negative (won’t show positive).
  • A Must Finish On constraint to assign our due date, the Total Slack on the critical path may be positive, negative or zero.

In addition to the critical path, near critical paths should be identified. Near critical paths are paths where the Total Slack value is close to the Total Slack value for the critical path.

Think about this scenario:

You are managing a twelve month project of approximately 300 tasks. When you create your schedule model, 9 paths are defined from the start of the project through the end of the project. When you identify the critical path, you see that it has a Total Slack of 2 days. This means that your current Finish date is 2 days before your commitment date.

If you decide to proceed with this plan without making any adjustments, what do you think the likelihood is of completing this critical path (and therefore the project) on time? In most cases, with this length of a project, if nothing is done to increase the Total Slack to something greater than 2 days, the chances are very close to 0%.

However, what if you have two additional paths that each have 4 days of Total Slack. What do you think the likelihood is of completing those paths on time? I would argue that they are probably also close to 0%, particularly if there are tasks in those paths with Duration estimates where you are less confident of the accuracy. These are what we refer to as near-critical paths.

______________________________________________________________________________

We recommend for each project that you manage, that you determine an acceptable level of Total Slack for likely success. We call this your Critical Task Threshold.  This threshold will vary from project to project, but should be based on things such as:

  • Total Duration of the project (or remaining duration if the project is in progress) – If you have a 12 month project, you would want a higher level of Total Slack on critical and near-critical tasks before you commit to a date, than if you have a 3 month project.
  • Confidence in your Duration/Work estimates for the project – Are the tasks in this project similar to things you have done in the past or are you developing new technology that you have no experience with?
  • Historical information from Project Closeout/Lessons Learned sessions for similar projects – Looking at past projects, how close do you typically come to meeting your original plan/baseline?
  • Other schedule risk factors – These may include things like external dependencies, resource availability, company reputation, etc. Some organizations will build schedule contingency into a project to minimize these risks, but if your organization does not plan schedule contingency, make sure to consider these factors when determining your Critical Task Threshold level.

Once you determine what the Critical Task Threshold should be for your project, the project should not be committed to, and baselined until ALL tasks have Total Slack values that are greater than this threshold.  Once the threshold has been met by all tasks you are ready to baseline and begin the project.

During project execution, you should continue to monitor and address ALL tasks that don’t meet the threshold, not just the very worst case critical path.

As the project progresses, the Critical Task Threshold should be reduced, as there is less of the project to complete.

The next lesson, Analyzing Critical Path Information in Microsoft Project, describes ways to focus in on your critical and near critical tasks, the tasks that don’t meet the Critical Task Threshold, to determine where to compress your schedule.

PreviousNext