The critical path is generally defined as the longest continuous sequence of activities in a schedule — it defines the project's earliest completion date, or minimum duration. Activities on this path are termed critical path activities. Once the forward and backward pass are complete, the critical path is confirmed by examining total float.
Total float is the time an activity can slip before its delay affects the program end date. When the network is free of date constraints, critical activities have zero total float — meaning any delay to a critical activity causes a day-for-day delay in the forecast finish date.
If an activity on the critical path is delayed by one week, the program finish date will be delayed by one week — unless the slip is mitigated by adjusting other activities, resources, or logic.
This is why the critical path is the most powerful tool a planner has for prioritising management attention and resources. The critical path assists program management in focusing effort where it will have the most positive effect on project performance.
Typically, scheduling software marks as critical all activities with zero or less-than-zero total float.
Once the early and late dates have been derived through the forward and backward pass, total float is calculated as:
TF = Late Start (LS) − Early Start (ES)
or equivalently:
TF = Late Finish (LF) − Early Finish (EF)
The result tells you the amount of time the activity can be slipped without delaying the project end date. A TF of zero means the activity is on the critical path.
Activities with total float within a narrow range of the critical path total float are called near-critical — they can quickly become critical if their float is consumed by even a small delay. Near-critical paths need only a small delay to become critical.
The near-critical float threshold depends on the project duration. A 5-day threshold may be appropriate on a short project; a multi-year programme may use a 2–3 month threshold. The principle is the same: any delay to near-critical activities can rapidly push them onto the critical path.
Parallel critical paths and near-critical paths with the potential to become critical must be identified, understood, and actively managed. Assessing the percentage of activities on the critical and near-critical paths gives a measure of schedule risk — a higher percentage places greater pressure on achieving the project's end date.
The sequence of activities that forms the critical path changes as the project progresses. Activities can shift on or off the critical path as:
Activities that were previously critical may become non-critical, and activities that were not critical may become critical. This is why the schedule must be regularly updated and the critical path re-evaluated at every update cycle.
It is crucial that program management understands that an important activity is not necessarily "critical." At any point in time, the critical path may or may not contain activities that management considers particularly significant.
A delay in an activity may be important for scope or cost reasons without delaying the finish milestone. Conversely, some mundane activities — training, permit approvals — may sit on the critical path and will delay the program finish date if they take longer than planned, even if they seem low-risk. The term "critical" in scheduling has a specific technical definition that should always be used consistently in reporting.
Test your understanding of float concepts. Select the best answer for each question.
Q1. What does "float" represent in a project schedule?
Q2. Total float refers to the amount of time an activity can be delayed:
Q3. Free float is most practically relevant for activities on the:
Q4. An activity with zero total float has:
Q5. Knowing the free float of an activity helps project managers:
Q6. In your own words, explain the difference between total float and free float in a construction project schedule.
Q7. Describe a scenario in construction scheduling where knowing the total float of an activity would be more important than knowing its free float.