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The critical path, also called the longest path, is a sequence of tasks that directly affect the project’s finish date. Each of the tasks along the critical path are called critical activities. If any of these activities are delayed, the whole project will be delayed. Critical activities must be completed by their planned finish date in order to be sure that the project will finish on time. The critical path is composed of the project activities with the least amount of float.
A CPM schedule is a project management activity schedule that uses the critical path method of scheduling. A CPM schedule shows the specific activities that make up the larger project, usually in graphic form. This includes the durations, sequential relationships, and any associated costs. CPM schedules provide the schedule with the least amount of time necessary to complete the project. This is done through calculations of activity and project data, including optimal start and finish dates for each activity.
What is the Critical Path Method?
The critical path method (CPM), also referred to as critical path analysis (CPA) is a scheduling algorithm that was developed in the late 1950s. CPM is a method of mathematical analysis that can be used by any project that has a sequence of activities. The critical path method is commonly used in construction, development, engineering, and research projects.
The critical path method works by using network diagrams. Network diagrams display the activities needed to complete your project in a certain sequence. The CPM calculates the project’s critical path, which is the least amount of time it will take to complete the project. It does this by using the activities’ durations, sequences, and the relationships between each one. This determines the duration of the project. The critical path also identifies the project’s critical activities. Critical activities are those that need to be completed at a certain time to ensure that the project is completed 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 Microsoft Project, can perform CPM scheduling automatically.
Why is CPM Scheduling Important for Project Management?
Using the Critical Path Method on your project’s schedule can provide valuable insight into the planning process. It also ensures that the schedule sticks to the necessary timeline. CPM Schedules will help you to identify all the tasks needed for the project and how long each task will take. Through the calculation of float, CPM can also show you which activities can be delayed, 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
This can give you a clearer insight into which activities are critical and must be completed on time, and which have float and can be delayed.
Facilitates in resource planning
Critical activities can be prioritized with assigned resources to ensure that they are finished on time, rather than non-critical activities that have some leeway.
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, these planned dates can be compared with actual dates. This can show you whether the schedule is ahead or behind the baseline you created.
Improves future project planning
Project managers can analyze variance data found between actual and planned dates to create more accurate estimates for future project plans.
Helps avoid potential project delays and ease project efficiency
It can help improve future project planning. You can use the CPM network diagram to get a better idea of which activities have dependencies, which can be run in parallel, and which need to be performed sequentially.
CPM Schedule Elements
Before getting into how the critical path method of scheduling works, we should go over some key elements involved within it.
Tasks, also called activities, are portions of the project that have an identifiable start, duration, and finish date. Tasks are the work that your team needs to complete to finish the project. They can be connected to a sequence that shows when the team should perform each activity. You can also give tasks dependencies. This means that some other task, a dependent task, must be completed before the task can occur. For example, a “Final Grade” task can have a dependency on a “Rough Grade” task. This means that you have to complete the “Rough Grade” task before you can complete the Final Grade task. The CPM algorithm will fuse the task durations, sequences and dependencies together to identify the project’s critical path. In other words, it fuses together all the elements of the tasks to give a timeline of the shortest possible duration of the project.
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 Software uses days by default. Durations are determined before an activity begins. Actual durations can differ from planned durations if unforeseen circumstances or delays occur.
Dependencies show how tasks relate to each other within the project as a whole. For example, dependencies can show whether tasks are going to be performed sequentially (one after the other) or 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. Dependencies are shown in CPM schedules as relationships, AKA logic or ties. Relationships define the sequence and dependencies between two tasks. CPM scheduling uses activity relationships and durations to identify the project’s critical path.
The critical path, also called the longest path, is a sequence of tasks that directly affect the project’s finish date. Each of the tasks along the critical path are called critical activities, and if any of these activities are delayed, the project as a whole will be delayed as well. Critical activities must finish by their planned finish date in order to ensure that the overall project finishes on time. The critical path is composed of the project activities with the least amount of float.
Float, also known as slack, is the amount of time that a task can be delayed without affecting the next task– and therefore the overall project. If a task has float, it can be delayed for a certain amount of time without necessarily delaying the rest of the schedule. This is known as a non-critical task. The activities on the critical path will have no float. This means that they cannot be delayed. CPM scheduling calculates activity floats to find activities that are critical and non-critical.
Earliest Start Date
The “earliest start date” is the earliest date that each task can start in the project. The earliest start date for the first activity in the project is usually the overall project’s earliest start date as well. The CPM algorithm will calculate early start dates for each activity to indicate the earliest date that each can begin.
Latest Start Date
The “latest start date” is the latest possible date that each task can start in order to complete the project on time. If an activity starts past its’ late start date, the following activity and the project as a whole could be negatively affected. The CPM algorithm will calculate late start dates for each activity to indicate possible dates that each can begin.
Earliest Finish Date
The “earliest finish date” is the earliest date that each task can be completed in relation to the overall project. The CPM algorithm calculates early finish dates for each activity to indicate the earliest date that each can be completed.
Latest Finish Date
The “latest finish date” is the latest possible date that each task can be completed 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 negatively affected. The CPM algorithm calculates late start dates for each activity to show the latest possible date that each can start.
How Does the Critical Path Method of Scheduling Work?
CPM scheduling works by running through the sequence of activities twice. During this time, it assigns early and late start dates for each activity. Although most scheduling software programs perform the critical path automatically, you can also do it manually.
To perform the critical path method for 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 completion. 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, activity 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.
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. Early dates can be calculated manually using the following calculation:
Early Start Date + Activity Duration – 1 = Early Finish Date
In the following example, let’s say that the project is planned to begin on January 1st. This will act as the early start date for the first activity, activity A.
According to the CPM algorithm calculation, activity A will have a duration of 2 days. If it starts on January 1st, it will have an early finish date of 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 date of January 5th, which allows activity C to have an early start date of January 6th. Finally, Activity E can start once 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.
Next, the CPM algorithm will do a backward pass through the schedule. It will start with the last activity and move backward through each activity. Along the way, it will assign each activity with a late start and finish date. These will be the latest dates that the activities can occur. Late dates can be calculated 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 CPM 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 on January 12th.
The critical path method algorithm will assign each of these activities with a late start and finish date.
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 shows the amount of time that an activity can be delayed without delaying the entire project. The critical path is composed only of critical activities. Critical activities are activities without float. Because of this, it is important to identify which activities have float and which do not.
Total Float is calculated as the difference between an activity’s late and early dates. Total Float can be calculated 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 calculated late and early finish dates to calculate the total float for each activity:
Activities A, D, and E have the same early finish date as their late finish date. This gives each activity a 0 float. This means that the activities are critical activities. 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 float. The same is true for activity B.
The CPM scheduling method uses the calculated early and late dates to determine which activities have float(non-critical) and which do not (critical). We are able to identify the critical path following the sequence of activities without float.
How to Build a CPM Schedule
Now that we’ve gone over how the critical path method of scheduling works, let’s go over the basic steps required to create a CPM schedule. Please note that many project management softwares, such as Primavera P6, will walk you through these initial steps. Once completed, the application uses the CPM algorithm to schedule dates and total float— as seen in the examples above.
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, you can determine which activity will start the project and which will follow. Activities can have more than one relationship tie and you will need to determine which activities have dependencies and which can be performed 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 that you should consider when tying activities together— 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 be performed sequentially – one after the other.
Start to Start (SS)
When one activity starts, the following activity will also start. This does not mean that the activities will finish at the same time, but just that they will be performed in parallel.
Finish to Finish (FF)
When one activity finishes, the following activity will also finish. The start of the activities do not have to be the same. At least part of these activities will be performed in parallel.
Start to Finish (SF)
When one activity starts, the following will finish. This activity type is uncommon, but is used when the start of one activity drives the finish of another.
The Critical Path Method will only work if the project’s activities are tied together logically. A perfect CPM schedule requires 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 the project management software. If you are attempting to manually create a CPM, 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:
Primavera P6 is used by governments, pharmaceutical companies, and many of the world’s largest construction companies. In fact, most scheduling related jobs by larger firms or governments require Primavera P6 in their contract. This is because Primavera P6 not only ensures efficiency, but tracks who is responsible for delays to the project. This is useful in keeping stakeholders accountable and filing claims if necessary.
Please note that most project management software automatically applies the Critical Path algorithm to the added activities when your project is scheduled. With these software 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.*Please note that many project management softwares, such as Primavera P6 or Microsoft Project, will do this for you automatically once activities have been identified and tied together with relationships. It is highly recommended to utilize a software application when performing CPM scheduling, as it will be immensely faster and more thorough than trying to perform it manually.
Estimate Activity Durations
Next, you will need to estimate how long it will take for each activity to be completed. This generally can be estimated through planning with team members and using data from previous projects. It’s important to remember that these are just estimates – 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 essentially the activities in the schedule that 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 calculations covered before finding each activity’s early and late start date. Once these dates are found, the total float for each activity can be calculated through the difference between those dates. The sequence of activities with 0 float will show the critical path.
*Please note that most project management software automatically applies the Critical Path algorithm to the added activities when your project is scheduled. With these software 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 are started and completed, the critical path may adjust on its own due to changes in start and completion dates. 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 from the steps above, you can create your schedule and perform critical path analysis much more efficiently using project management software. Without software, the critical path method requires a lot of manual, time-consuming work. This includes performing individual calculations on each activity, and drawing out a network diagram. There is also a much higher chance of performing scheduling 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 setting important task details. For example, dependencies, relationships, and durations. Additionally, these programs will automatically create a network diagram via the Gantt Chart. This will calculate the critical path with just a clock of a button.
At Taradigm, we offer the premier CPM scheduling application, Primavera P6. Primavera P6 is used by governments, pharmaceutical companies, and many of the world’s largest construction companies. In fact, most scheduling related jobs by larger firms or governments require Primavera P6 in their contract. This is because Primavera P6 not only ensures efficiency, but tracks 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 with training. At Taradigm, we offer Primavera P6 training with a live, experienced instructor, and self-paced online video training. For those who would like to outsource their scheduling to experts, we specialize in CPM training, scheduling, and claims work, and have decades of experience.
For those who are moving their business to the cloud, we are pleased to offer Oracle Primavera Cloud (OPC). OPC is the next evolution of Primavera P6 and is more affordable, easier to set up and maintain, and offers more features than Primavera P6. We also offer comprehensive training for OPC for Primavera P6 veterans and those new to scheduling.
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Lauren Hecker is an Oracle Primavera Cloud and Primavera P6 Instructor and teaches 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!