Critical path activities are the project tasks that must start and finish on time to ensure that the project ends on schedule. A delay in any critical path activity will delay completion of the project, unless the project plan can be adjusted so that successor tasks finish more quickly than planned.
How are critical activities identified?
You can choose between two options for identifying activities as critical: float and longest path.
- Float is a measure of schedule flexibility.
P6 Web Access uses the Critical Path Method (CPM) to generate a project schedule. This method calculates four dates for each activity in the project plan: Early Start, Late Start, Early Finish, and Late Finish. If the Early Start date and Late Start date for an activity are the same, the activity is said to have zero float. Activities that have zero float must start on time to prevent the schedule from slipping.
Critical activities can also have negative float. Negative float occurs when an imposed finish date creates a schedule that is shorter than the duration calculated to complete the activities on the critical path. A project with negative float is behind schedule.
To meet the specific requirements of your project, P6 Web Access lets you specify the float tolerance used to identify activities as critical. For example, in some situations you might want critical activities to be those with zero or negative float. In other situations, activities with float of three days or less might need to be identified as critical.
- Longest path defines the sequence of driving activities
that determine the project end date.
The longest path is broken when activities are no longer driven by relationships; that is, when activity dates are driven by constraints or resource leveling. Longest path calculation includes interproject relationships. Therefore, activities designated as on the longest path may change depending on whether you schedule a project alone or with its related projects. If a project has interproject relationships and you schedule it alone, the interproject relationships are treated as scheduling constraints.