The critical path method (CPM) scheduling was formally developed in the late 1950’s to address the challenges of better planning, scheduling and managing complex projects. It is the most-used scheduling technique in the construction industry and is also used extensively in aerospace, software development, product development and engineering.
By modeling activity durations, logical relationships and the end point (completion milestone) of the project, CPM scheduling allows one to determine which activities are “critical” to the completion of the project. With the term “critical activity” meaning those activities that must be performed on their earliest planned start and finish dates to avoid causing a delay to the overall end date of the project. By definition, the critical path of the project is the longest path through the network that establishes the minimum overall project duration. Thus, each time the schedule is updated the critical path can be be calculated to determine the anticipated project completion date. Knowing which activities are critical allows the project team to stay focused on progressing the work along the critical path. Knowing if the end date has slipped allows the project team to analyze the critical path and come up with a recovery plan to accelerate the work along the critical path. Critical path analysis is also used to perform delay analysis and to justify time-extension requests. Non-critical activities can be monitored to ensure they do not become critical and potentially delay the project.
For the classical schedule calculation, the critical path is the continuous chain of activities through the network schedule with zero total float. Total float is an important characteristic of CPM scheduling because it indicates the amount of time an activity can start from its planned start date without delaying the project. Total float and the critical path can be identified using the following parameters:
- Forward Pass:
- Calculates an activities “early dates.”
- ES – Early Start: The earliest date an activity can start based on the completion of its predecessor activities.
- EF – Early Finish: The earliest date an activity can finish based on the ES date.
- ES + Duration – 1 = EF
- Backward Pass:
- Calculates an activities “late dates.”
- LS – Late Start: The latest date an activity can start without delaying the end date of the project.
- LF – Late Finish: The latest date an activity can finish without delaying the end date of the project.
- LF – Duration + 1 = LS
- Total Float:
- The amount of time an activity can slip from its early start (ES) without delaying the project.
- The amount of time between an activity’s late dates and early dates.
- For classical schedule calculations, activities with zero total float are critical
- LS – ES = Total Float
- LF – EF = Total Float
Every time you schedule a project using CPM scheduling software such as Oracle-Primavera, the scheduling algorithm performs a forward and backwards pass and calculates the total float for each activity. You cannot edit an activities float value directly. Primavera allows users to choose whether to calculate the critical path by total float or by the longest path. The difference between the longest path and total float is that the longest path calculation will always identify the activities that are truly critical and driving the end date of the project. Total float calculations can sometimes be affected by constraints and differences in project calendars that can yield total float values where the path(s) with zero or negative float may not be the true critical path of the project. Therefore, it is always recommended to schedule a project using the Longest Path and not Total Float when determining the critical path of the project.
Diligent monitoring of the critical path is key to keeping your team focused on timely project delivery and allow them to quickly address issues and delays as they occur. Disregarding the critical path can lead to costly delays and delay claims.
Carolina Consultants is a Raleigh, NC based consulting firm providing a variety of CPM planning, scheduling and forensic delay analysis services. For true expertise from a reliable, experienced source, trust Carolina Consultants to help you with your CPM scheduling needs.