This recommended practice (RP) for Identifying the Critical Path is intended to serve as a guideline and a resource, not to establish a standard. As a recommended practice of AACE International it provides guidelines for the project scheduler when reviewing a network schedule to be able to determine the critical path and to understand the limitations and assumptions involved in a critical path assessment. Such a determination is a part of the total cost management (TCM) project planning, scheduling forecasting, and change management processes.

This RP focuses on the types of prospective analyses performed in real-time while the project is underway, the advantages and the limitations of each in identifying the critical path of network schedules. The identification of the critical path and understanding what calculations and settings went into making that identification is a critical first-step in the schedule analysis process. Other recommended practices discuss analysis of schedule trends and the issues involved with plan changes intended to modify the critical path.
The work breakdown structure (WBS) is used to break down (or decompose) a project into all component activities necessary to deliver a complete and functional project for the stakeholders. The critical path method (CPM) is used to apply logic to determine sequencing of those activities to form an activity “network,” and then mathematically determine activity timing. The mathematical calculation determines the earliest time that an activity can start in a calculated “forward pass” through the network. The latest time that an activity can start is determined by calculating the network in a “backward pass.”
There are various accepted methods for determining the critical path. This RP describes four such CPM methods and explains the calculations used in these methods. It assumes a working understanding of the CPM calculation process.

This RP also discusses the analysis of the output of a CPM calculation. There is no absolute standard or definition of “proper” CPM calculation procedures or a list of required or optional rule extensions. No professional organization or standards testing bureau currently evaluates, certifies or validates CPM software algorithms or procedures. Identifying the critical path requires an understanding of the methods and algorithms used by the various software platforms in producing the CPM calculations.
Different CPM software products can produce different calculation results when applied to the same schedule. A project scheduler must understand the differences and nuances of the software tools they will most often encounter and be prepared to account for the same in their analysis. In addition, different CPM calculation options from the same CPM software can produce different results for the same schedule logic. This RP provides an analysis of the output and identifies the strengths and weaknesses of the processes used to obtain those results.

This RP acknowledges that identification of the critical path is likely made via various software products. Specific software is identified in this RP in order to qualify these methods and algorithms. In the interests of accuracy, only those platforms where the authors have expert knowledge are cited here. This does not constitute a recommendation of any particular company or software product. References to other software platforms may be added to this RP as they become available.