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What is a CPM Schedule in Construction - A Beginner's Guide to Schedule Analysis

Updated: Sep 11, 2021

For construction project managers, the CPM (Critical Path Method)’s main function is to help everyone understand how individual tasks fit into the project schedule network and affect project duration. But that’s not all they might gain from the CPM.

Project scheduling isn’t only about planning dates for tasks. It’s also about matching resources and equipment with tasks. This way we might greatly eliminate downtime (for example, waiting for workers to arrive). Procurement must also be planned ahead of time so that resources arrive when needed. (When resources arrive too early, it can mean added costs for storage and maintenance.)

While paying attention to the Critical Path (CPM) may help ensure that projects finish on time, going one step further to analyze the critical path and apply schedule compression techniques may actually *decrease* the project’s total duration.

Fast tracking and crashing, for example, reduce waste caused by waiting for preceding tasks to be completed, perhaps because there are bottlenecks in the system. Proactive management may remove some of these bottlenecks by adding equipment or resources and by realizing that sometimes a preceding task does not have to be 100 percent complete before the successor task can begin.

Project owners, especially of large, sophisticated projects, often demand that project schedules identify the critical path and that ancillary construction activities such as procurement and resource management, are conducted with the critical path in mind. This is why the CPM is required learning in most project management programs. Project delays can be costly for owners who are eager to begin generating revenue in new facilities. Many owners will request detailed CPM schedules 30 to 45 days before the project begins, so they may monitor the actual pace of work. 

Complicating the picture, is that field supervisors often aren’t fans of overly detailed formal scheduling. They tend to feel it isn’t relevant to actual, fluctuating field operations, and therefore see it as a distraction. They may even prefer short-term, in-the-field scheduling. 

But, beyond small, uncomplicated projects, CPM is essential.

Major construction has too many moving parts to track without a detailed schedule. Big construction firms typically use formal schedules for complex structures that require extensive coordination.

The widespread use of mobile devices and cross-platform software has also helped formal scheduling proliferate. Field supervisors are now easily accessing schedule information on mobile devices.

CPM requires a lot of math, and with all the different task required in a construction project 0 software take a lot of the pain out of the CPM.

Now you might simply input some basic information and let pre created software run algorithms for you.

Construction managers should also differentiate between two broad categories of scheduling techniques:

  • Time-oriented scheduling is the standard model, and focuses on project completion times as a function of precedence relationships. It’s the default approach for most scheduling software. 

  • Resource-oriented scheduling focuses on the effective scheduling and provision of resources. Construction planners typically call on it for projects where resources are limited to deal with those constraints. Otherwise, expensive bottlenecks may to emerge. 

It’s important to determine which type of scheduling is appropriate before starting scheduling.

Critical Path Method is a Time-oriented Technique and may be completely redundant for a project with limited resources - as the resource shortage becomes a more important hindering factor than the pace of work!

Time-oriented scheduling is generally the default approach, but many construction managers actually use both approaches - limiting specific resource use in otherwise time-oriented schedules. Scheduling software assists in building these models. 

Scheduling techniques can make varying usage of the float in a schedule network diagram.

Different graphical techniques are used to represent schedules. One option is a time-scaled schedule, where the schedule is scaled and activities are sized according to duration. Gantt charts, are a favorite. Some schedulers also prepare percentage complete charts, occasionally with lines to represent both as-early-as-possible and as-late-as-possible schedules.

With resource-oriented scheduling, planners often plot graphs for resource use. 

Taught widely at the university level, the CPM has continued to prove its worth as a scheduling technique despite some proven shortcomings

Warnings and Well-Dones for the Critical Path Method in Construction

Despite universal use of the critical path method, construction pros say there is much confusion around it, especially among owners and architects. Taking the maximum benefit from CPM requires common sense and proactive management practices. 

Firstly, the accuracy of the critical path model depends on the quality of information you feed into it. Incomplete or unrealistic data will lead to an inaccurate critical path analysis. In other words, you must use common sense when employing CPM.

Moreover, the CPM model can’t tell whether you have forgotten some activities or whether your duration estimates are accurate. Rather, you must employ careful attention to detail and critical judgment on the part of planners. Inaccurate estimates and omissions early in a project lifecycle can have a big impact, as one compounds the effects of errors.

Schedulers need to approach CPM with both theoretical knowledge and a realistic appreciation of life’s variability.

CPM also can’t identify tasks where you might decrease task durations by adding resources - this requires an experienced construction manager. 

Remember to check assumptions about resource availability, so you know whether resource-oriented scheduling techniques might be a more appropriate choice than purely time-oriented techniques. 

Insufficient understanding due to lack of expertise is another common obstacle of CPM. CPM execution suffers when schedulers do not have a complete understanding of the system as well as some construction field experience.

Successful implementation also requires support from top management. 

Another noted hindrance is lack of belief in the value of CPM and the consequential lack of budget and time to perform proper scheduling.

Estimating Departments are common place among Contractors, but Scheduling Departments (where you make sure work is done properly) are very rare!

CPM pros are also mindful that it’s not a one-and-done system. Conditions vary, projects change, and estimates are revised.

Users must update the critical path model to take these into account, so commitment to the method and following through are essential. 

Revisions are commonplace, expensive and time consuming. If the critical tasks aren’t affected, the changes shouldn't delay the project. But to know for sure, you have to go through the revisions process. 

Largely reflecting the degree to which it mystifies and intimidates those who have not mastered it, CPM in some circles has become little more than a contractual formality. To them, it’s useful for calculating project duration, and not for engaging in active management strategies, such as diverting resources during construction. 

Avoiding Inaccuracy

Inaccurate CPM schedules can often be tied to a few causes:

  • Underestimating the level of effort required to complete a certain task.

  • Lack of subcontractor feedback during the development of the baseline CPM schedule.

  • Lack of detail in the construction CPM schedule may prevent accurate modeling of the work.

  • Manipulation of project logic for a favorable position with regard to a potential claim, which may occur during the course of construction.

Construction Critical Path Method Best Practices

The following may prevent common mistakes with CPM schedules:

  • Learn from history.

  • Collaborate with the project/construction management team to plan in detail the way to proceed.

  • Model the entire scope of work, including owner responsibilities that may delay the project.

  • Model the procurement process in the schedule, so you can monitor submittals, approvals, ordering fabrication time, and delivery.

  • Model all non-project work that could affect completion of the project.

  • Within the limitations of the location, carefully model the plan for resource use, so the schedule shows a clear flow of resources throughout the project.

  • Provide enough detail in the schedule to use mostly finish-to-start logical relationships (i.e., one step must finish before the next can start). 

  • Allow adequate time for development of the schedule, quality control review, and claims avoidance review, as well as updates and analysis.

  • Use an integrated cost/schedule/risk approach with continuous risk assessment and monitoring.

Reminders Before Using Software for the Critical Path Method

Scheduling software has made calculating the critical path a lot easier.

These applications speed up critical path and reduce the risk of mathematical error. They can also quickly identify the critical path and critical tasks, and almost instantly process updates to the total project duration.

In addition, they figure out float times for non-critical activities.

Some software communicate possibilities for reducing task duration and help you to model the effects of schedule compression techniques like crashing and fast tracking. 

With mobile interfaces and cloud storage, software makes CPM charts easy to share and to access in the field. Both free and paid versions of scheduling software with CPM functionalities exist.

An enterprise resource planning solution may be helpful if the critical path affects more than a single project.

A Warning!

Scheduling experts warn that software can give a false illusion of CPM mastery. 

As it is now very easy to create these schedules, people may think that they have done everything they need to do in simply creating their schedule.

There is still a base of knowledge which is required - as with any software - to be able to make actually beneficial use of it.



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