Critical path

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A critical path is the sequence of scheduled activities that determines the duration of a project plan or an iteration plan. The critical path is the longest path through a project or an iteration, and determines the shortest time possible to complete the activities in the plan. The maximum of the earliest finish dates of all the work items in the plan is used to calculate the longest path through the plan. Activities on the critical path must be completed according to the planned schedule for the plan to be completed on time. If an activity on the critical path is delayed, the plan finish date is affected. Project managers can use the critical path to support their decisions as they prepare and track release schedules, plan for resources, and take remedial actions if a plan deviates from the schedule.

This feature is available only if you are using the Formal Project Management process template.

When you create or save a plan, the critical path is calculated automatically by evaluating all work items that are assigned to the plan. The critical path is always calculated at the plan level. For an iteration plan, the critical path is calculated by evaluating all work items for that iteration. For a release plan, the critical plan is calculated by evaluating all the work items in all the child plans.

When the plan changes, the critical path is recalculated.

For work items with an assigned resource, the critical path calculation depends on the following information:
  • Plan dates
  • Work item estimates
  • Constraints
  • Dependencies
  • Project area work environment
  • Resource calendar
  • Availability, including scheduled time, absences, and project allocation
To understand how the resource leveling process affects the scheduling process, see Basic resource leveling.
For work items without an assigned resource, the critical path is calculated by using the following information:
  • Plan dates
  • Work item estimates
  • Constraints
  • Dependencies
  • Project area work environment
  • Project-level settings, such as working time per day, quitting time, and absences
If the plan start date is in the future, the critical path is calculated by using that date. If the plan start date is in the past, the critical path is calculated by using the current date.

For each work item, the remaining time from the current date is considered for the calculation of the critical path. Only work items that are not started or are in progress can be on the critical path. Work items that are resolved have no remaining estimate, and are not on the critical path.

To calculate the critical path, the following four parameters are determined for each activity:
  • Earliest start date (ES): The earliest time at which the activity can start, based on the constraints and dependencies for the activity. The earliest start date can change as the plan progresses and changes are made to the planned schedule.
  • Earliest finish date (EF): The earliest start time for the activity plus the time required to complete the activity. The earliest finish date can change as the plan progresses and changes are made to the planned schedule.
  • Latest finish date (LF): The latest time at which the activity can be completed without delaying the plan, based on the constraints and dependencies for the activity.
  • Latest start date (LS): The latest finish time minus the time required to complete the activity

Based on these dates, the slack or float for an activity is calculated. The slack for an activity is the time between its earliest and latest start time, or between its earliest and latest finish time. The slack represents the amount of time that an activity can be delayed past its earliest start or finish without extending the finish date of the longest path in the plan.

If the slack is zero, the task or activity is critical and is highlighted in the Gantt chart view. On the critical path, none of the activities have slack, that is, for all these activities, ES=LS and EF=LF. A delay in an activity on the critical path delays the project. To accelerate a delayed project, reduce the total time required for the activities on the critical path.

A work item without dependencies can be critical in the following situations:
  • The duration of the work item is equal to the longest path in the plan
  • The work item finish date, which is calculated from the remaining time, is equal to the finish date of the longest path in the plan; for example, due to a Start no earlier than constraint
  • The same user is fully assigned to one or more tasks whose remaining time is equal to or longer than the longest path in the plan
If a work item with a dependency has slack from a predecessor task due to a Start no earlier than constraint, the slack is not considered because it does not allow scheduling flexibility. As a result, the work item is on the critical path.

During project execution, the critical path can change. Delays in activities outside of the critical path can affect the project finish date and cause those activities to be on the critical path.