Confirming the Critical Path
The critical path is generally defined as the longest continuous sequence of activities in a schedule. It defines the program’s earliest completion date or minimum duration. Activities on this path are termed “critical path activities.” Typically, the sequence of activities with the longest total duration is also the path through the network with the lowest 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 float, and therefore any delay in the critical activity causes the same day-for-day delay in the program forecast finish date.
For example, if an activity on the critical path is delayed by a week, the program finish date will be delayed by a week unless the slip is mitigated.
Therefore, the critical path is most useful as a tool to help determine which activities deserve focus and, potentially, management help. The critical path assists program management in prioritizing resources to have the most positive effect on program performance.
Typically, total float is set to zero, and the scheduling software marks as critical all activities with zero or less-than-zero total float.
Activities with total float within a narrow range of the critical path total float are “near-critical” because they can quickly become critical if their float is used up in a delay.
Near-critical paths need only a small delay of time to become critical. Management must monitor critical and near-critical activities through sound schedule
management because any delay in them will delay the entire program. Near-critical paths are monitored according to a float threshold tailored to the schedule.
For example, a brief schedule might consider a 5-day slip to be a near-critical threshold. In programs scheduled to take years, a 2- or 3-month’s slip in near-critical paths might make the path critical. Because prolonging a schedule by 5 days on a short project is as easily possible as prolonging a multiyear project several months, program managers should manage all near-critical and critical paths.
The critical path is not constant. The sequence of activities that make up the critical path changes as activities are delayed, finished early, occur out of planned sequence, and so on.
Activities that were previously critical may become noncritical, and activities that were not critical may become critical.
It is crucial that program management understand that an important activity is not necessarily “critical.” At any point in time, the critical path may or may not contain activities that management believes are particularly important.
A delay in an activity may be important for any number of reasons related to scope and cost without delaying the finish milestone date. In contrast, some mundane activities—training, for example—may be on the critical path and not particularly risky but can delay the program finish date if they take longer to accomplish.
Similarly, an activity of long duration should not be referred to as a “critical path activity” simply because it will take a long time to accomplish. “Critical activity” in scheduling parlance has a specific definition that should be adhered to when reporting and evaluating schedule data.
Total Float
Once the early and late dates have been derived, the schedule can be assessed for flexibility. The difference between the time an activity may start or finish and the time it must start or finish in order for the project to be completed on time is known as total float (TF). Total float is calculated as the difference between an activity’s early and late dates:
TF = LS – ES
or
TF = LF – EF
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