In project management, a CPM schedule is a schedule that uses the critical path method of scheduling. This is a method of scheduling in which users do not choose dates for tasks to start and finish. Instead, users will just enter in information about the tasks – such as their durations, calendars and relationships. Based on this information, the critical path method algorithm will calculate when each activity needs to start and finish to complete the project on time.

Many project management programs – like Primavera P6 and Primavera Cloud – use the critical path method by default. This is useful, as most project contracts require CPM schedules for approval. However, even if your scheduling program does all of this work for you, it can be important to understand how the critical path method works.

In this article, we will dive into what the critical path method is, how it works, and how to best use it in your project schedules.

What is the Critical Path Method?

The critical path method (CPM), also referred to as critical path analysis (CPA), is a scheduling algorithm developed in the late 1950s. CPM is a method of mathematical analysis used to calculate dates from a project with a sequence of activities. You’ll find the critical path method used in construction, engineering, and research projects. 

The critical path method works by analyzing your activities’ durations, calendars, and sequences of work. Using this information, CPM will calculate the project’s critical path, which is the least amount of time it will take to complete the project. Ultimately, the critical path determines the overall duration of the project. The critical path also identifies the project’s critical activities. Critical activities are those that have no float. In other words, these activities need to start and finish right on time to ensure that the project completes on time. Using the critical path and the Project Planned Start date, you’ll be able to determine what date each activity needs to start and finish on to complete the project on time.

In the past, the critical path method was often applied to projects by hand. Today, however, most scheduling softwares, such as Primavera P6 and Primavera Cloud, will perform CPM scheduling for you.

Why is CPM Scheduling important for project management?

Using the Critical Path Method on your project’s schedule can provide valuable insight into when each task needs to start and finish. Ultimately, it can help you and your team stick to a schedule to ensure the project completes on time. CPM Schedules tend to be more realistic – as you’re not entering in idealised Start/Finish dates for activities. Instead, the program calculates these dates for you, along with your float values. Not only will this show you when activities need to start and finish, but it can also show which activities you can delay, if needed.

Critical Path Analysis can benefit project management and planning for the following reasons:

Helps you to identify important task details 

This includes dependencies, risks, constraints, and duration estimates.

Allows you to prioritize activities based on their float periods 

In CPM schedules, you have clear insight into which activities are critical, and which activities have float. You can potentially delay activities with float without delaying the project’s completion. Because of this, you can put more effort into ensuring your critical activities are performing as needed.

Facilitates in resource planning 

Differentiating between critical and non-critical activities can help with resource planning. You can prioritize assigning resources to critical activities to speed them up. If you have limited resources, you can ease allocation on non-critical activities to free them up.

Eases project monitoring 

The CPM schedule can show whether an activity is on time or late according to the original schedule. It will provide you with planned early and late dates for each activity. When the project actually starts, you can compare these planned dates with actual dates. This can show you whether the schedule is ahead or behind your original project plan.

Improves future project planning

Using this variance data, project managers can make more accurate estimates for future projects. For example, if the last project fell behind schedule based on a lack of resources, you may want to hire more resources for the next job.

CPM Schedule Elements

Before getting into how CPM scheduling works, we should go over some key elements involved within it.

Tasks

Tasks, also called activities, are the individual pieces of work that you need to do in order to complete the project. In CPM scheduling, task dates are typically determined by dependencies, or relationships. Dependencies determine the order in which activities will occur. The CPM algorithm will use the task durations and dependencies to identify task dates and the project’s critical path.

Task Durations

Every activity/task will need to have a duration. Durations are the amount of time required to complete the tasks. You can track durations using different units, such as hours, days, weeks, or months. Most scheduling programs, like P6 Professional, use days by default. Schedulers will estimate these durations before the activity actually begins. Actual durations can differ from planned durations if unforeseen circumstances or delays occur.

Task Dependencies

Dependencies, or relationships, determine the sequence in which the tasks should be performed. Additionally, they show whether tasks will perform sequentially (one after the other) or in parallel (at the same time). Often, one task may require another task to start or finish before it can begin. For instance, an activity for Strip Concrete Footings may only begin once an activity for Pour Concrete Footings finishes. You can add dependencies to CPM schedules using predecessor and successor relationship ties. CPM scheduling uses activity relationships and durations to identify dates and the critical path.

Critical Path

The critical path, also called the longest path, is the sequence of tasks that affect the project’s finish date. Tasks along the critical path are critical activities. Critical activities must start and finish directly on time to ensure that the project finishes on time. If these activities are delayed, the overall project completion will be delayed as well. The critical path is typically composed of project activities with the least amount of float. 

Float

Float, also known as slack, is the amount of time that you can delay a task without affecting the project’s completion. Tasks with float are known as non-critical, and they have some leeway to when they can start and finish. The activities on the critical path have no float, and are known as critical. If critical tasks are delayed, the overall project completion will be delayed. CPM scheduling calculates float values using the activity’s early and late dates.

Early Start Date

The early start date is the earliest date that each task can start in the project. The early start date for the first activity in the project is usually the overall project’s start date. The CPM algorithm will calculate early start dates for each activity to indicate the earliest date that they can begin.

Late Start Date

The late start date is the latest possible date that each task can start to complete the project on time. If an activity starts past its’ late start date, the following activity and the project as a whole will be delayed. The CPM algorithm will calculate late start dates for each activity to indicate the latest date that they can begin.

Early Finish Date

The early finish is the earliest date that each task can finish within the project. The CPM algorithm calculates early finish dates for each activity to indicate the earliest date that each can finish.

Late Finish Date

The late finish date is the latest possible date that each task can finish for the project to finish on time. If an activity finishes past its’ late finish date, the following activity and the whole project could be delayed. The CPM algorithm calculates late finish dates for each activity to show the latest possible date that each can finish.

How does the Critical Path Method of Scheduling work?

CPM scheduling works by performing both a forward and a backward pass through your schedule. Through these passes, each activity will be assigned early and late start and finish dates. Most scheduling programs perform the critical path method automatically – but you can also do it manually.  

To perform the critical path method on your project, you will first need a list of defined tasks with durations and relationships. Tying together activities and relationships will give you a sequential order of work. Let’s look at the following project example:

In this example, the sequence of activities begins with activity A, which has a duration of 2 days. When activity A finishes, activities B and D will begin. Activity B has a duration of 3 days, and activity D has a duration of 10 days. When activity B finishes, activity C, with a 1 day duration, will begin. After D and C have completed, activity E will begin. Activity E has a duration of 5 days.

The Critical Path Method will run through these activities twice to assign each activity early dates and late dates. 

Forward Pass

First, the CPM algorithm will run a forward pass through the schedule. It will start with the first activity and then move forward to assign each activity an early start and finish date. These early dates are the most optimistic start and finish dates for your project. You can calculate early dates manually using the following calculation:

Early Start Date + Activity Duration – 1 = Early Finish Date

In the following example, let’s say that the project will begin on January 1st. This will act as the early start date for the first activity, A.

According to the CPM calculation, activity A will start on January 1st and finish on January 2nd. This means that activities B and D can begin the following day. This gives them both an early start date of January 3rd. Activity B has an early finish of January 5th, which allows activity C to have an early start of January 6th. Finally, Activity E can start once both C and D have finished. Although activity C is set to finish on January 6th, activity D will not finish until January 12th. This means that E’s early start date is January 13th.

This is how the critical path method of scheduling works to assign early start and early finish dates to each activity in the schedule.

Backward Pass

Next, the CPM algorithm will do a backward pass through the schedule. It will start with the last activity and move backwards through the relationships. Along the way, it will assign each activity a late start and a late finish date. These are the latest dates that the activities can occur. You can calculate late dates using the following formula:

Late Finish Date – Activity Duration + 1 = Late Start Date

In the following example, we will use the early finish date for the last activity in the project as the late finish date.

The calculation will start with the last activity, activity E. Activity E has a finish date of January 17th and a late start date of January 13th. The previous activities, C and D, would have been completed the day before, on January 12th.

The critical path method algorithm will assign each of these activities with a late start and a late finish date in this same manner.

Total Float & the Critical Path

Early and late dates are both required to identify the project’s critical path. To find this path, the CPM algorithm calculates Total Float for each activity in the project. The Total Float value is the amount of time that you can delay an activity without delaying the project’s completion. The critical path is composed of only critical activities. Critical activities are activities without float. These activities have no room for delay, and have to start and finish directly on time. Because of this, it is important to identify which activities have float and which do not. 

Total Float is the difference between an activity’s late and early dates. You can calculate Total Float using either of the following formulas:

Late Finish Date – Early Finish Date = Total Float

Late Start Date – Early Start Date = Total Float

In the following example, we will use the late and early finish dates to calculate the total float for each activity:

For activities A, D, and E, the early finish date is the same as the late finish date. This means that these activities have 0 days of float – making them critical. Activity C’s late finish date was January 12th and its early finish date was January 6th. There is a difference of 6 days. This means that activity C has 6 days of total float. The same is true for activity B. 

Through the critical path method, we now have both dates and float values for each activity in the schedule. Although you can calculate these values on your own, most scheduling programs – like Primavera P6 and Primavera Cloud – will do all this work for you.

How to Build a CPM Schedule

Now that we’ve gone over how CPM scheduling works, let’s go over the basic steps required to create a schedule. Many project management softwares, such as Primavera P6, will walk you through these initial steps. After building the schedule, your scheduling application will use the CPM algorithm to schedule dates and calculate total float.

Building a CPM schedule requires the following steps:

Identify the activities

First, you will need to break the project’s work into identifiable activities. You can create these activities from the project’s scope, or through a Work Breakdown Structure (WBS). It is often useful to associate specific activities with unique details to differentiate them. For example, unique names, IDs, and categorical codes.

Identify dependencies and create relationship ties

Next, you will need to sequence and connect activities. You can do this by identifying dependencies and creating relationship ties. Using the activities you created, determine which activity will start the project and which ones will follow. Activities can have more than one relationship tie – but ideally each activity should have at least one predecessor and one successor. You will also need to determine whether these relationships will perform sequentially, or in parallel.

Different relationships can connect activities in different ways. For example, you can tie two activities together in a Start-to-Start relationship. This means that the start of one activity is dependent on the start of another. Other activities can be tied together in a Finish-to-Start relationship. This means that the start of one activity is dependent on the completion of another activity.

There are four different relationship types available for you to use. You’ll choose the relationship type depending on whether the start or finish date of one activity is driving the start or finish of another.

Finish to Start (FS)

When one activity finishes, the following activity will start. These activities will perform sequentially – one after the other.

Start to Start (SS) 

When one activity starts, the following activity will also start. The activities will start on the same date and perform in parallel – at the same time. One activity may finish before the other – all that matters is that they start on the same date.

Finish to Finish (FF) 

When one activity finishes, the following activity will also finish. The start dates of the activities do not have to be the same. These activities will finish on the same date and will work in parallel.

Start to Finish (SF) 

When one activity starts, the following will finish. This activity type is uncommon, but can be used when the start of one activity drives the finish of another.

The Critical Path Method will only work if you tie the project’s activities together logically. CPM schedules require every activity to have both a predecessor (an activity that precedes it) and a successor (an activity that follows it). The only exceptions are the first and last activities in the schedule. The first activity will only need a successor and the last activity will only need a predecessor. 

Create the Critical Path network diagram*

This step is only required if you are not using a project management application. If you are manually creating a CPM schedule, you will want to create a network diagram or chart of linked activities. This will act as a visualization of the activities and relationships added so far. 

The following is an example of a network diagram:

*Many project management softwares, such as Primavera P6, Primavera Cloud and Microsoft Project, will do this for you automatically. We recommend using a software application when performing CPM scheduling, as it will be much more efficient.

Estimate activity durations

Next, you will need to estimate how long it will take for each activity to complete. You can generally estimate this using data from previous projects. It’s important to remember that these are just estimated durations. In reality, these activities may take more or less time, depending on the circumstances. 

Use the Critical Path algorithm to identify the Critical Path*

You will now need to find the critical path. The critical path is the sequence of activities in the schedule that are critical – or cannot be delayed. It is important to identify the critical path because it indicates the longest duration in the project. You can identify the critical path using the calculations above to identify each activity’s early and late dates. After identifying these dates, you can calculate the total float value for each activity as the difference between the early and the late dates. The sequence of activities with 0 float will show the critical path.

*Most project management programs will automatically apply the Critical Path algorithm to the added activities. With these programs, there is no need for manual calculations. The early dates, late dates, total float, and critical path for your project are immediately identified by the program.

Update the CPM schedule during execution

Once your project has started, you can make updates to the critical path as needed. As activities start and finish, the critical path may adjust due to delays. If there are any unexpected events or delays to the schedule, you may need to manually adjust the critical path. You can do this by compressing or incorporating additional activities into the CPM schedule. It is important to continue to analyze and update your CPM schedule to make sure that the project is working as efficiently as possible.

CPM Scheduling Softwares

As seen above, you can create your schedule and perform critical path analysis much more efficiently using project management software solutions. Without software, the critical path method requires a lot of manual, time-consuming work – like calculating dates and drawing diagrams. There is also a much higher chance of schedule errors, which can cause complications during the project. If any CPM date or total float calculations are incorrect, the entire project could fail as a result.

There are a variety of different project management softwares available that can help you with CPM project planning. Many of these will assist in breaking the project down into tasks and adding task details – like relationships and durations. Additionally, these programs will automatically create a network diagram via the Gantt Chart. Lastly, with just a click of the button, these programs will run the critical path method on the schedule for you – eliminating the need to perform manual calculations.

At Taradigm, we offer the premier CPM scheduling applications, Primavera P6 and Primavera Cloud. Primavera P6 and Primavera Cloud are used by governments, pharmaceutical companies, and many of the world’s largest construction companies. In fact, most scheduling jobs by larger firms or governments require Primavera P6 or Primavera Cloud in their contract. This is because Primavera P6 and Primavera Cloud not only ensure efficiency, but track who is responsible for delays to the project. This is useful in keeping stakeholders accountable and filing claims if necessary.

Many contracts also require proficiency in Primavera P6 and Primavera Cloud with training. At Taradigm, we offer Primavera P6 and Primavera Cloud training with a live, experienced instructor. For Primavera P6, we also offer self-paced online video training. For those who would like to outsource their scheduling to experts, we specialize in CPM scheduling, TIAs, and claims. Please feel free to contact us for more information.

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Lauren Hecker is an Primavera P6, Oracle Primavera Cloud, and Microsoft Project instructor. She teaches both onsite and virtual scheduling courses. To see her next open enrollment course, please visit our calendar. To schedule an onsite or custom course, please contact us!