What is the Critical Path Method in Project Management

The Project Critical Path is the longest path of activities that must be completed in order for the project to be completed. All activities that are on the critical path, if late, will cause delays in the project.

Project managers need to know the critical path as it allows them to:

  • Determine the length of the project duration in a more reasonable way.
  • Identifying dependency relationships between activities, constraints, and risks on the project.
  • Establish a realistic project schedule based on work priorities.

Project managers utilize the critical path method (CPM) algorithm to define the shortest amount of time required to finish each task with the least amount of float.

Project scheduling software equipped with Gantt charts can now automatically calculate critical paths, making the CPM method much easier.

Now that we know about the critical path, we can learn more about the critical path method (CPM), which is based on this concept.

What is CPM and Why Do We Use the Critical Path Method (CPM)?

Project managers use the critical path method (CPM) to create a project schedule and estimate the overall project duration.

The critical path method is a project schedule and estimation technique that’s used to create an accurate timeline of the total duration of a project.

This technique uses a network diagram, which visually represents task sequences or tasks needed to complete the project. Once these are identified, the time frames are calculated to identify the longest path over all paths and its overall time frame.

Critical Path according to PMBOK® Guide 5th Edition:

The sequence of activities that represents the longest path through a project, which determines the
shortest possible duration.

History of CPM

Morgan R. Walker and James E. Kelley created the critical path method in the late 1950s.

The two different project management techniques are CPM and PERT.

PERT is suitable for projects where it is not known how long the required activities will take to complete, whereas CPM is appropriate for recurring projects which have a set timeline.

What are the Benefits of Using CPM in Project Execution?

Tasks in a project must follow a set schedule to stay on track by a set deadline. If you don’t do this, the scope of your project will easily run out of control.

The critical path technique is particularly significant in project management since it defines all the tasks required to finish a project. It determines what work must be completed on time which can be delayed if necessary, and how much float or slack there will be.

When done right, critical path analysis can help you in the following ways:

  • Estimate the duration of each task more accurately.
  • Determine task dependencies, resource constraints, and project risks.
  • Prioritize task based on their time float or slack, which helps with project planning and resource allocation.
  • Identify important work that cannot be delayed and ensure that it is carried out on schedule.
  • Monitor the progress of your project and track any deviations from the timeline.
  • Compressing the schedule (on certain activities) after estimating the project completion time in the beginning.

Important Factors of CPM

We must understand the concept of CPM and the variables that affect it before learning how to calculate the critical path with this method.

Relationships between activities: Finish to Start, Finish to Finish, Start to Start, Start to Finish.

Early Start (ES): this is the earliest start time for an activity in your project. You can’t know until you identify the task dependencies between activities in the project.

Late Start (LS): is the latest time an activity can be started before affecting the project delay.

Early Finish (EF): The earliest finish of an activity that will not affect the overall project schedule.

Late Finish (LF): The latest finish of an activity that will not affect the overall project schedule.

Float: Float, often known as slack, is the word that defines how long a task can be delayed before it impacts the task sequence and project schedule. Since tasks on the critical path cannot be deferred, they have no float (float value 0).

Let’s look at some critical path examples to understand these parts of critical path analysis better.

Critical Path Example

Example by HBR

The following is an illustration of a CPM diagram. It might help you visualize the importance of critical paths to the project timeline, even at a high level.

CPM-Diagram
Source: Wikipedia

For now, we will use this critical path diagram to demonstrate the components of the CPM approach.

Harvard Business Review provides an example of a critical path schedule.

How to Find a Project’s Critical Path in 8 Steps

Now that you understand the basic idea of ​​the critical path approach let’s see how to calculate the critical path in 8 phases:

1. Compile Project Activities.

To collect all project activities that contribute to the final result, use a work breakdown structure (WBS).

You can use Primavera to collect activities.

Collect-activities
Activities in Primavera

2. Determine Dependency Tasks (Relationship).

Determine which tasks depend on other tasks before starting them. Use your knowledge and feedback from your team members. Failure to correctly identify the relationships between activities makes the critical path method ineffective.

Primavera or Microsoft Project has features to make it easier for you to implement relationships between activities.

Activities-Relationships

3. Create a Critical Path Diagram.

A critical path analysis chart, often known as a network diagram, displays a sequence of activities.

Primavera or Microsoft Project has the option to display activities in Network Diagram mode.

Primavera-Netword-Diagram

4. Define a Timeline.

You must estimate the duration of each task to use the critical path method. Utilize data from previous projects and other sources of knowledge, such as from Engineer Experts related to the work.

The following is a display of the date and duration of the Primavera. You can use the duration scale in days, weeks, or months.

Duration

5. Utilize the Critical Path Algorithm.

The critical path algorithm is divided into two parts: Forward Pass and Backward Pass.

Forward Pass

Determine the Early Start (ES) and Early Finish (EF) of each activity using the network diagram and the planned duration of each activity.

The activity’s ES is the same as its predecessor’s EF, and its EF is determined by the formula EF = ES + activity duration. Estimated time required to complete the entire project identified by EF from the last activity.

Backward Pass

Start by setting the Early Finish of the last activity as the last finish. The formula for calculating LS is LS = LF – duration (activity). For the previous activity (Predecessor), LF is the smallest start time for the next activity (Successor).

All the above calculations can be performed automatically in Primevara once you access the following menus: Tools ->Schedule… (F9) -> Schedule.

Scheduling-Menu

6. Define float or slack for each activity.

Using this formula, you can calculate the float or slack of each task.

Float = LS – ES.

In Primavera, Total Float is automatically calculated. Critical Path consists of activities that have the smallest Total Float value (usually 0).

Total-Float-Activities

7. Determine the Critical Path.

The critical path consists of activities with a float value of 0. Except for the first activity in your CPM schedule, all of these critical path activities are task-dependent. All project tasks with positive slack run concurrently with critical path activities.

Critical-path-Primavera-in-Red-Line

8. Make Changes During Execution.

As you go through the execution phase, keep updating the critical path network diagram according to the actual data.

These critical path analysis steps determine which activities are critical and which have floats or can be delayed without affecting the project schedule.

You now have the knowledge you need to design critical path schedules more precisely and have a better chance of meeting your project deadlines.

Modifications or other constraints that may affect the project schedule should also be considered. The more unexpected events or risks you can anticipate, the more accurate your critical path schedule is.

If time is added to the project due to this constraint, it is referred to as the critical path drag, which is the additional time the project will take as a result of the activity and constraint.

Now look again at the schedule in the following table:

CPM-Table-1

The relationships that can be formed from the activities in the table can be seen in the image below using the Precedence Diagram Method (forward pass and backward pass):

CPM-Path
Image Source: Project Scheduling and Control (ebook) by Saleh Mubarak.

From the image above, it can be seen that the critical path is marked with a bold black color.

Also, read what is the Gantt chart.

Multiple Critical Paths

It may not be common to have more than one critical path, but it is possible for this to happen.

There are a few reasons why there could be more than one:

  • Different paths have the same duration.
  • Milestones in a constrained project reflect phased handovers/completions/contractual requirements which cause different paths to be critical at the same time.
  • Individual schedules, each with their own critical path, combined into an overall master program.

As long as you understand why there are multiple paths and what they’re all about then that’s ok!

Project Management & CPM Software

Project scheduling software did not exist when the critical path method (CPM) was invented, so project managers had to calculate the critical path manually.

Fortunately, several project management software options are available today that can help with critical path processes.

Primavera P6 and Microsoft Project are two of the most frequently used project management software for identifying critical paths.

Here are some of the key features you’ll need as a project manager to effectively use the critical path method for scheduling:

  • Add Activity (start date and duration).
  • Creating relationships between activities.
  • Create Calendar.
  • Displaying float.
  • Add Constraints.
  • Display critical path with different color.
  • Adding resources.
  • Saving baseline.
  • Actual updates.
  • Comparing actual and baseline.
  • Doing filtering.
  • Showing S-Curve.
  • Tracking Historical for schedule change (in Primavera).
  • Input Budget.
  • Reporting.

Determining Critical Path on Primavera P6

At Primavera P6 there are two options for determining Critical Path, namely based on the number of Total Floats or based on the longest path in the activities network.

Define-Critical-Path-in-Primavera

To determine a critical path, you can access:

Tools >> Schedule (F9) >> Options

Once the critical path has been determined, close the Schedule Options window and click on the Schedule button.

Example Using Microsoft Project

You may have made a series of simple activities schedules related to a project. Suppose you have a program or plan to move to a new house.

The following are alternative activities planning that you can make:

1 – Contacting the moving house transport service – 1 day.

2 – Determine the day (discussion with family) – 1 day.

3 – Determine the items to be carried and left – 3 days.

4 – Make a checklist of goods – 1 day.

5 – Cleaning the new house before moving – 1 day.

6 – Install a new home air conditioner – 1 day.

7 – Install a new house trellis – 2 days.

8 – Transport heavy goods on moving day – 1 day.

9 – Transport of light goods – 1 day.

10 – Check if there are items left behind – 1 day.

11 – Contact the old homeowner – 1 day.

12 – Tidying up in the new 1 – day house.

13 – Install new home curtains – 1 day.

14 – Clean the new house – 1 day.

15 – Decorating a new house – 1 day.

To create the schedule above, you first have to determine the relation between each activity.

After that, you can determine the sequence of all activities.

Selection of the critical path can be made by looking at the sequence of activities that form the longest path.

From the schedule that has been made above, the critical path can be determined as follows:

Critical-Path

It can be seen that on the path marked in red color, the total duration resulting from activities on that path is greater than the total duration generated by another path.

Conclution of Critical Path

From the examples above, a conclusion can be made about the Critical Path, which is the longest path of the schedule which consists of a series of activities from the beginning to the end of the project.

Here are some observations about critical paths:

  • In every schedule, there must be at least one critical path.
  • The critical path can be more than one but it is not recommended.
  • Each critical path must be continuous from start to finish of the project.
  • Exception: when a constraint is set, the path may be critical from the start to the constraint activity or from the constraint activity until the end.
  • Some people define the critical path as a path with a total float of zero. This is true as long as there are activities with a total float of zero from start to finish.
  • Some of them define the critical path as the longest path on the schedule (from start to finish). The definition of the critical path is the longest path is more precise in most cases because if the project finish date is made longer, the path with a total float zero will change to positive (more than 0). So the critical path will be the longest path with the least number of total floats.
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