About Critical Path Method (CPM)

The Critical Path Method (CPM) is a project management technique used by the Architectural/Engineering/Construction (AEC) industry to plan and manage projects large and small. CPM was first implemented in the 1950’s. Since then, it has become the standard project management tool for the AEC industry.

Just as AEC projects are a team effort, so is the development and management of the effective CPM network. CPM provides the project team with a common perspective of what needs to happen, by whom, and by when. It is different than Gantt Chart (Bar Chart) scheduling in that the schedule is calculated twice: once from the beginning of the project to the end, and then from the end of the project to the beginning. In this way, two sets of dates are determined: the earliest that an activity can start/finish, and, perhaps more importantly, the latest that an activity can start/finish without jeopardizing the overall project completion date.

The basic CPM terms are Early Start and Early Finish (collectively known as Early Dates), Late Start and Late Finish (collectively known as Late Dates), and Total Float (the amount of time that an activity can float between the Early and Late Dates without delaying the project completion date). The “Critical Path” is the chain of activities that control the project completion date.

Float Time

On a typical AEC project, many project activities possess some amount of float time (i.e., not every activity needs to happen as soon as it possibly can). However, confusion over how much float time an activity has can lead to significant conflicts between the project team members (Owner, Architect, General Contractor, Subcontractors, and other project stakeholders). Proper use of the use of the critical path method can reduce this inherent confusion, and produce consensus.

The available Total Float can be used to efficiently allocate resources or to accommodate unforeseen events or scope changes. Total Float is generally considered a shared resource between team members. Once the Total Float is consumed, additional planning adjustments (relationships and/or duration estimates) must be made or the project completion will be delayed.

Project Management Basics: Balancing the Inherent Conflict Between Scope-Cost-Time-Resources

All project management endeavors seek an efficient balance between the scope (work required under the terms of the contract), the project budget, the time allowance, and an efficient resource allocation (manpower, equipment, material). Proper use of the Critical Path Method can help the project team formalize and communicate that desired balance.

Conflict and Expectation Management

Conflict is inherent on AEC projects. Project stakeholders often have different needs and expectations. The owner may expect the project to be completed on-time and on-budget. The general contractor has to finish within the allotted time, ensure its profit margin, manage relationships with subcontractors, and manage its relationship with the owner. Subcontractors require an efficient work environment so that productivity factors can be maintained. The project’s end user -- the ultimate client -- may require additional programming changes so as to efficiently pursue its business goals. All of the team members must balance their interests against the clock. Time is money.

CPM is Not Intuitive

To become effective in the application of CPM, teams must become educated in the science and the art. Implementing the Critical Path Method is a science in that the CPM network calculations are precise and repeatable. It is also, however, an art in that there are a number of choices that the team must make. For example, the team must decide how to describe the project activities in the CPM network, and determine how the activities in the CPM network relate to one another.

What About CPM Software?

There are a number of commercially available software programs available to assist in the preparation and management of CPM schedule networks. Among them are Oracle P6® and Microsoft Project®.

Calculating the CPM network with a pencil is simple. Unfortunately it is also extremely time consuming. The commercially available software tools are specifically designed to quickly (almost instantly) calculate the CPM network.

The old saying, "Garbage in equals garbage out (GIGO)," is very relevant when it comes to using the CPM software. If the information entered into the software is flawed, then the calculated Early Date, Late Date, and Total Float calculations will also be incorrect.

The CPM software also allows the team members to organize project activities in a variety of ways. By presenting the information in an efficient manner, the team members can more easily “see” the project from their perspective. For example the drywall contractor may wish to see his activities in relation to the painter, by floor and area, with all other trades activities removed from the view.

There is a Method to This

We can help your organization implement effective use of the Critical Path Method. We can facilitate the development of CPM schedules for a specific program or project, and we can help your organization become proficient in the science and art of effective CPM scheduling.

Contact Us.