Just as programmers are divided between those who start counting from 1 and those who start from 0, planners and schedulers are split between defining the Critical Path via ‘Longest Path’ and ‘Total Float’. But who’s right?

Both methods for defining the Critical Path have their advantage and drawbacks. Let’s explore them.

The Longest Path method is beneficial because it includes all activities that directly influence the project’s completion date, even if some of these activities have a Total Float greater than zero. However, this method focuses on the final project end date, which is not always the most critical concern. Project stakeholders are more interested in the delivery of key milestones rather than when the project officially concludes. They need to know the Critical Path in the context of project commitments, typically called deadlines.

The Longest Path method also struggles when a schedule has ‘Start No Earlier Than’ (SNET) constraints that may require to reflect existing limitations. In such cases, the Longest Path can be reduced to a few activities or even one milestone, which may be theoretically correct but practically ineffective.

The Longest Path method has one more critical limitation. Practically, we know that activity duration forecasts are not precisely 100% accurate. This is a risk that an activity formally doesn’t belong to Longest Path but practically drives project completion day. For instance, an activity with just one hour of Total Float may not technically be on the Longest Path, but it must be managed as if it is critical.

The Total Float method addresses some of the Longest Path method’s limitations, especially when the schedule includes Deadlines and SNET constraints. It can also widen the definition of what is considered critical by including near-critical activities. For example, activities with Total Float of less than 8 hours could be considered critical, allowing for more flexible oversight.

However, this method struggles when multiple calendars are used in the schedule. In such cases, activities on the Longest Path can have a Total Float greater than zero, even though they impact the delivery date. It is essential to identify such activities, as it may be possible to accelerate the delivery date by adjusting the calendar or resource assignments.

My recommendation: use both, but do it smartly.

The theoretical definition of the critical path with the longest path may be technically correct but impractical for day-to-day project control. The ultimate base should be Total Float.

However, when schedule optimisation is performed, it is useful to know the current longest path as it may be possible to accelerate delivery by shortening an activity with a positive Total Float.

Analysing the project from both the Longest Path and Total Float perspectives gives a better understanding of critical activities and potential risks.

As an option, planners can develop a filter that covers critical activities regardless of applied methods. For that, they can calculate critical path based on Total Float including near-critical activities

and create a filter that shows all activities that are Critical or Longest Path.