Introduction to the Critical Path Method (CPM)
The Critical Path Method, or CPM, is a scheduling technique that identifies the longest path of logically dependent and connected activities that it takes to reach the end of the project or the end project deliverable. Although there can be more than one critical path, otherwise known as secondary or tertiary critical paths, there is only one primary critical path. Any delay in the project’s critical path will prevent the project from completing and delivering on time.
Tasks on the critical path can be identified in the schedule along with the Gantt portion of the schedule. These critical activities not only can be identified but also micromanaged to ensure that the project completes on time. Before understanding the critical path activities and how to find them, it is imperative to understand the basic requirements to set up a project schedule that is conducive to a solid critical path. This is all part of the CPM methodology that project managers, planners/schedulers, and other project team members use to manage their projects. The CPM is especially important in the construction industry, where delays in even small tasks can translate into catastrophic and costly project delays. The CPM and its visualization is important and is used in a wide variety of industries.
Before demystifying the critical path, the project team must ensure that the execution project schedule is set up properly. Numerous standards make up an ideal project schedule, and we are going to present some of the main requirements in this article.
CPM Requirements and Initial Project Set-Up
A solid project schedule will contain a detailed breakdown of activities in the form of tasks to define the scope of work in a project to drive to an end delivery or goal. These tasks should contain realistic durations and sufficient descriptions of the required work scope, and they should be logically linked. All tasks in a project schedule with exception of the first task (often a start milestone) and the final task (often a finish or delivery milestone) should contain both a predecessor and a successor task relationship. The majority of the task relationships will contain finish-to-start relationships, which means that a task cannot start until its predecessor task(s) have been completed.
Project resources in a resource-loaded schedule should be properly defined and assigned. When a project schedule contains resources, whether it is personnel or material or both, for example, the project team must correctly assign those resources to the appropriate tasks. Also, the project team must ensure that the correct project calendar has been set up in the project schedule. This project calendar will determine how the tasks and associated resources fall within the timeline of the schedule. Project calendars may consider the company, regional, or national holidays, as well as employee vacation schedules. Depending on the project schedule’s execution location or individual company schedules, a project can contain more than one calendar. Different calendars can be assigned to different resources and tasks within the project schedule.
Finally, the execution project schedule must contain a baseline, as this is the only efficient way to monitor a project’s progress against its original delivery agreements. Once the project team has thoroughly reviewed and agreed to the project schedule and its delivery and critical dates have been established, the project schedule should then be baselined before its execution. Then, the current date or status date can be set and the project schedule can go into kick-off and execution.
Finding the Critical Path in Primavera P6
Now that the execution project schedule is properly set up and baselined, we are ready to review the critical path. In the following example, we will use an execution schedule that is already in progress and that already contains status updates. In our sample schedule, we will first go to the Tools tab in Primavera P6 and select “Schedule…” or use the F9 key after we have opened our project schedule in Primavera P6:
When the Schedule box launches, select the “Options…” button:
Once the Schedule Options box launches, select the option “Total Float less than or equal to” under “Define critical activities as”. Ensure that it is set to zero and then select the Close button (activities with zero float are considered critical):
The red bars represent the critical path in the Gantt portion of the project schedule:
You can also use the filters option in Primavera P6 to sort out only the critical path activities. Use the filter button in Primavera P6’s top layout menu to select “Critical” under the Default options. Click “OK”:
Only the critical path activities are now displayed:
Another way to view the critical activities in Primavera P6 is to right-click on the schedule and select “Columns…”:
Select “Critical” under the General heading on the left side of the Columns screen and use the arrow in the middle to bring that selection to the Selected Options box. Select “OK”:
Primavera P6 shows you which tasks are critical in the schedule:
Project schedules can have more than one critical path; however, there is risk in a project containing multiple critical paths. Multiple driving dependencies can cause more than one critical path in a project schedule, although the project team will find more efficiency in managing a single critical path.
The Forward and Backward Passes Relating to the Critical Path
Primavera P6 can automatically calculate a project schedule’s forward and backward passes while considering the critical path. The forward pass is calculated by analyzing both the Early Start and Early Finish dates. An Early Start (ES) date is the earliest time and date that an activity can start, provided that its predecessor requirements have been completed first. The Early Finish (EF) date is the earliest time and date that the activity can complete.
In contrast, the backward pass calculation analyzes both the Late Start (LS) and Late Finish (LF) dates. The Late Finish, as you likely figured out, is the latest time and date that an activity is required to start without impacting and delaying the project schedule. Similarly, the Late Finish date is the latest time and date that the activity can complete without delaying the project schedule.
Go to Primavera P6’s top layout menu and select the Activity Network icon:
Primavera P6 displays the entire activity network:
Mechanicals showed up earlier in our exercise as key activities that are on the schedule’s critical path, so we have clicked on this section to focus further on these activities. The red lines represent the critical logic ties between activities, while the dashed lines represent non-driving relationships (these relationships are not required drivers of the critical path):
Right-click in the activity network and select “Activity Network Options…” from the pull-down menu:
In the next screen, we want to find the selection for displaying the Early Starts and Early Finishes in the activity network. We do this by using the pull-down arrow in the Activity Network Options dialog box:
Select “<Custom>” from the pull-down menu and select the “Box Template” button at the bottom of the dialog box:
In the Chart Template box, click the “Add” button to choose the options for BL Project Duration, Early Start, and Early Finish as illustrated below. Then click “OK”:
Click “Apply” and then “OK”:
The Primavera P6 network diagram now contains important information regarding each of the dependencies on the critical path, listing the Early Start and Early Finishes for each of the critical tasks:
The backward pass is shown in the same manner in the activity network. Following the same steps as you did for the forward pass, use the “Add” button to select the Late Start and Late Finish in place of the Early Start and Early Finish that you selected before:
You can see the layout that you will see for each activity in the Activity Network Options dialog box. Select “OK”:
Primavera P6 displays the activity network with the backward pass details. You will notice that the dates have changed for some activities, as the backward pass shows the latest dates that the critical path activities must start and complete without delaying the project schedule:
Float Calculation and the Critical Path
The difference between the late dates and the early dates in Primavera P6 can be thought of as the calculation of the float. Keep in mind that the project schedule may contain free float, which is similar to a built-in cushion of time where an activity can be delayed without impacting and delaying its successor activity’s completion date.
The term “free float” applies to the amount of time that an activity can be delayed without the project completion being impacted and delayed. This is why it is imperative to keep track of the project’s critical path and the health of the critical activities that comprise it.
ScheduleReader and the Critical Path
ScheduleReader allows you to review the critical path similarly to Primavera P6. ScheduleReader is not only compatible with scheduling and planning software such as Primavera P6, but its functionality mimics the ease and visual appeal of Primavera P6. Once the project schedule is imported into ScheduleReader, the critical path activities are represented by the red bars in the Gantt chart:
Go to the Filter icon in the Activities tab to launch the Filters dialog box. From here, you can check the “Critical” option and select “OK”:
ScheduleReader now shows only the activities on the critical path:
Finally, there is another way to view the critical path activities with ScheduleReader. Under the Activities tab, select the Activity icon and choose “Critical” under the General section (remember this from our demonstration in Primavera P6) on left and use the arrow in the center to move the selection to the Inserted Columns box:
A column is now present that expresses whether or not the activities in the schedule are critical:
Efficiencies for Team Collaboration Using ScheduleReader and Critical Path Analysis
CPM can help the project manager and the project team manage their projects and identify risks to the project deliverables. Utilizing ScheduleReader offers the project team similar and convenient features that can be found in Primavera P6 for team collaboration, such as its ability to easily display the critical path and use flexible and customizable layouts that can be viewed easily from an imported Primavera P6 XER file, whether the user has access to Primavera P6 or not.
This function allows cross-team collaboration while keeping all members of the project team current on the project schedules and risks to critical deliveries. Project team members benefit in large operations where expensive Primavera P6 licenses may not be possible for all users. ScheduleReader offers the same benefits of utilizing the CPM to the project team members who do not require access to Primavera P6 functionality for updating purposes to the project schedules.
About the Author
Melanie Calverley is an experienced professional with several years of knowledge and practice in Project Management as a Program Cost and Schedule Control Analyst on multi-million/multi-billion-dollar projects, Project Controls, Strategic Planning, Engineering Planning, Earned Value Management (EVM), and Earned Value Management System (EVMS) implementation, Configuration Management, and writing/editing. She commands full utilization of Primavera P6 software, MS Project, and MS Project Server. Her career background includes industry experience in oil and gas, energy, aerospace/defense, IT, litigation, and media.
Calverley has extensive experience in the planning and scheduling arena since 1999, as well as extensive experience in the aerospace and oil and gas industries combined. Calverley has worked for the large and high-visibility players in the aerospace industry - Boeing and Lockheed Martin, as well as the large companies in the O&G industry, such as Schlumberger, GE Energy, and Chevron. She supported NASA directly and a host of smaller sub-contractors over the years before exploring the O&G and IT industries. Calverley understands the strict, organized flow of the aerospace industry's horizontal and vertical logic integration, the criticality of resource loading, and the reporting functions that support Earned Value Management (EVM). She has also participated in various audits such as DCMA audits, JSRs, ISRs, and CAM reviews. In 2013, Calverley was responsible for implementing a full EVMS in Primavera for Cameron International's (now Schlumberger) Process Systems Division across the globe. The system was fully tested and successfully implemented and run in the United States, Brazil, the UK, Malaysia, Singapore, and the Middle East.
Calverley owns her own business, Calverley Consulting, LLC, which has been operational since 2017. Calverley is also a solid writer, editor, and process flow documentation expert. She possesses a full command of the proper English language, and she is also a published book author. Calverley developed and championed multiple process flows and written documentation via policies and procedures for various well-known companies.