Critical Path Analysis
The critical path is defined as the longest logical path through the CPM network and consists of those
activities that determine the shortest time for project completion. Activities within this or list form a series
(or sequence) of logically connected activities that is called the critical path. A delay to the start or
completion of any activity in this critical path results in a delay to project completion, assuming that this
path consists of a continuous sequence of activities without an overriding date constraint or multiple
calendars.
Normally, there is only one critical path though a network schedule. There may be multiple critical paths
through a schedule. Multiple critical paths may occur due to different paths having exactly the same
overall duration. Constrained milestones (perhaps reflecting contractual requirements) may cause
different paths to be critical at the same time. Individual schedules (each with their own critical path) may
be combined into a master schedule that has a different critical path. In the case of a master schedule,
there are both individual schedule critical paths as well as a larger program critical path. Project schedule
specifications should define the legal interpretation of each type.
Another type of critical path is called a “resource critical path.” As there are rarely unlimited resources
available to the project team the constraint of limited resources often strongly influences the critical path
calculation. Therefore, in addition to the standard CPM calculations and logical connections, there is an
additional, implied constraint of limited resources. In a resource critical path, after CPM network
calculations have been made activities are further delayed, interrupted, or accelerated to reduce the
overall daily unit resource requirement to some predetermined resource limit. This may be accomplished
by adding preferential (or soft) logic, by manual or automatic resource leveling, or just by the physical
resource limits imposed or utilized on the project without prior planning. While many aspects of standard
critical path analysis are pertinent to resource critical path analysis, this RP does not further address the
subject of a resource critical path. This RP does not address schedules that contain settings that force the
activity durations to be calculated by resource availability, as in resource-driven schedules.
Successful project schedulers must determine and communicate activities that are critical, as well as
those that are near-critical (see later discussion in “Near-Critical Activities/Paths”), in a CPM schedule.
Management by exception[1] processes dictate that project management should devote more attention to
maintaining the required production level for activities on the critical path than for other activities further
removed from criticality. Further, recognition that duration estimates are not absolute and susceptible to
variation requires that a critical path analysis should also include the review of near-critical paths. Both of
these topics are addressed in more detail later.
The list of critical path activities often changes from schedule update to schedule update as activity status
is revised and the schedule is recalculated. Any activity associated with the CPM schedule that is delayed
long enough can eventually become critical, regardless of its original status.
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