The Critical Path and the Longest Path
The critical path is theoretically the sequence of activities that represents the longest path between the program’s start and finish dates. If the program has started, then the critical path will extend from the program’s current status date to the program’s forecasted finish date. In reality, however, as a schedule becomes more complex, total float values may not necessarily represent a true picture of the number of days an activity can slip. For example, multiple calendars, out-of-sequence progress, date constraints, and leveled resources can all produce misleading values of total float in complex schedules, leading to a misrepresentation of the sequence of activities that actually drives the program finish date.
As we noted in Best Practice 2, date constraints may cause activities to become critical, regardless of the total float that may be available if not constrained in the network. Specifically, backward-pass date constraints on activities other than the finish milestone will influence the criticality of activities. Hence, where constraints are many, there may be many more activities with zero or negative total float than activities that are actually driving the key program completion milestone.24
Figure 29 shows in the construction schedule two sequences of activity necessary to complete interior rough-in: critical activities (in red) and noncritical activities (in blue). The critical path is also the sequence of activities that represents the longest path to the “interior rough-in complete” milestone. The sum of the durations of the critical activities is 13 days. That is, the minimum duration from the start date of “install exterior doors and windows” to the “interior rough-in complete” milestone is 13 working days. A second path of activities, consisting of “rough in interior HVAC and through-roof penetration” and “inspect rough-in HVAC,” after the installation of the exterior doors and windows, has 2 days of available float and is therefore not considered critical.
Figure 29: The Critical Path and the Longest Path
Figure 30: Critical Path Activities Not on the Longest Path
The network now has 2 more critical activities than the network without the date constraint and shows 2 parallel critical paths in terms of total float. However, the longest path—in terms of duration—remains the same, regardless of the date constraint. While “inspect rough-in HVAC” is marked critical in the schedule, it actually has 2 days of relative float because it can slip 2 days before causing the “interior finishes complete” milestone to slip. The longest path is also referred to as the driving path because it determines the date of the key milestone. A driving path can be identified for any key milestone or activity to determine the sequence of activities driving its finish date.
When the critical path is not the longest path, the longest path is preferred because it represents the activities that are driving the sequence of start dates directly affecting the estimated finish date, if we ignored the presence of date constraints. Therefore, rather than simply filtering on activities that are marked critical by the scheduling software, management should be aware of the activity “drivers” that are determining the schedule finish date.
Moreover, driver activities may or may not have the lowest total float values when activities other than the program finish milestone have date constraints. Continuing with the framing example in figure 30, suppose the general contractor mandates the inspection of the rough-in HVAC by Thursday, November 20 (figure 31).
Figure 31: The Longest Path and the Lowest-Float Path
The lowest total float path consists now of “install exterior doors and windows,” “rough in interior HVAC and through-roof penetration,” and “inspect rough-in HVAC.” However, neither “rough in interior HVAC and through-roof penetration” nor “inspect rough-in HVAC” is determining the finish date of the “interior rough-in complete” milestone. Most scheduling software calculates activity drivers along with critical activities.
Negative float and its causes are discussed in detail in Best Practice 7.↩︎