Critical Path Analysis

Karen Toms

March 24th, 2020
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Critical Path Analysis (CPA), also known as the Critical Path Method (CPM), is a powerful project management tool which has been used extensively for decades in the construction industry.

CPA assists you in defining and planning all of the tasks which are required in your project and how dependent they are on each other.

It’s especially valuable for planning and managing larger construction projects which involve multiple subcontractors or a wide range of tasks, for example a series of different properties for one housing development.

If you find that your current method of managing projects is not giving you a clear picture of how to manage your resources most efficiently, CPA would be a valuable addition to your project management toolbox.

What is CPA?

CPA has been described by the Harvard Business Review as “a powerful but basically simple technique for analysing, planning and scheduling large complex projects.” The Project Management Body of Knowledge defines CPM as ‘the sequence of scheduled activities that determines the duration of the project.”

Basically, CPA involves listing out all of the critical and non-critical tasks in a project, together with the minimum and maximum length of time each task will take to complete.

This allows you to clearly see the maximum amount of time that the whole project will take and the interdependencies between the different tasks which will need to be completed.

As a result, you will be able to see how you can use resources more efficiently, where you might need to scale up on subcontractors or free up resources for other elements of the project, where you can save time, deliver tasks ahead of schedule or get a project back on track.

How to Identify the Critical Path

First you need to define all of the tasks in your project and the order they need to be completed in.

Start with defining the critical tasks. These are the ones which will need to be completed to ensure that the whole project is not delayed.

For example, in a house build project, the critical tasks will include laying the foundations, building the framework, and installing the roof. It is essential that these types of task happen in a set sequence with each task needing to be completed before the next one begins.

Below is a simplified flow chart example to demonstrate how the process works. Don’t worry, you won’t need to do this by hand as there are many different software tools which will do the legwork for you. MS Project is the most well-known but there are many other options out there.

Figure 1 – Critical Tasks


Figure 1

Non-critical tasks are those tasks which are not dependent on the completion of any other tasks. This means that you can do them before or after a certain stage is completed. Going back to the house build project, examples of non-critical tasks include installing drainage, plumbing, and electrics.

Figure 2 – Critical & Non Critical Tasks

Figure 2

Once you have mapped out all of your critical and non-critical tasks, the next step is to look at each task and estimate key timeframes to establish how long each will take. This will enable you to work out the length of the whole project. Starting from the beginning, estimate how long each of the critical and non-critical tasks will take. This will then allow you to work out the earliest and latest start and finish date for each task. In the diagram below you will see the following abbreviations:

  • ES – Early Start: This is the earliest time that a task can start, given that previous tasks must be completed first.
  • EF – Early Finish: This is the earliest finish time for the task.
  • LS – Late Start: This is the latest start date that the task must be started without delaying the project.
  • LF – Late Finish: This is the latest time the task must be completed without delaying the entire project.

The latest start or finish for one task will become the earliest start or finish for the subsequent task in the path.

Figure 3 – Establishing the Critical Path

Figure 3

The critical path is the path through your project in which none of the tasks have been delayed i.e. the path for which ES=LS and EF=LF for all tasks in the path.

This exercise will give you the length of the critical path in days and therefore, an overall timeframe for the project. It will also give you two important durations – the minimum and the maximum duration of your project.

If a task on the critical path is delayed by a week, the whole house build project will be delayed by a week. Similarly, in order to speed up the completion of the house, you will have to achieve one or more of the tasks along the critical path quicker than you have estimated.

As you are unable to vary the order of tasks on the critical tasks path, if you want to reduce the project schedule, you will need to answer a range of different practical and budget related questions.

Examples could include: Is it physically possible to reduce the time needed to complete critical task by, for example, assigning more contract labour to the job, working overtime, using different equipment etc? If so, would the cost of speeding up the task be less than the saving achieved by reducing the overall project time?

Defining Slack

Being able to define the amount of ‘slack’ in a project is one of the key advantages of using CPA.

Slack is the amount of time that a task can be delayed in a project from its early start without causing a delay to the overall project timeframe. Slack is also sometimes referred to as float in project management.

In Figure 3, we can see that for some tasks their early start is equal to their late start, while for others this is not the case. The difference between a task’s early start and late start (or between its early finish and late finish) is called its slack.

The amount of total slack in a project is defined as the maximum amount of time a task(s) may be delayed beyond its early start without necessarily delaying the project completion time.

Figure 4 – Defining Slack

Figure 4

Figure 4 completes the full CPA picture. It shows us how much slack there is for each task of the project.

You will see that tasks that are not on the critical path have a difference between their early start date and their late start date, i.e. those tasks could be delayed without affecting the project completion date. The slack in those activities is called free slack. The house developer could examine these tasks further to decide the best way to make use of this slack to save time and money in their project.

The Benefits of Using CPA

There are a multitude of benefits which you can gain by using CPA for your project management. Here are some of key ones:

Greater Resource Efficiency – Arguably CPA’s biggest benefit is that it will help you allocate resources on your project more efficiently, which should in turn allow you to complete it as quickly and cost effectively as possible. For example, if your construction firm is building several houses on one development, CPA will allow you to schedule not just the first build, but the programme across the whole site, giving you the advantage of managing resources efficiently and moving contractors around the site in the most optimum way.

Time Tracking – By providing you the tools to be able to visualise the whole project, what needs to happen when, which tasks can be moved in the schedule, how to do things quicker, how to avoid delays etc, CPA provides a way of effectively tracking time realistically across your project.

Decreased Delays – Identifying the critical sequence of tasks in your project will help you to identify ways in which you can minimise delays by scheduling work in the most efficient manner throughout your project.

Cost Calculations – CPA is far more valuable than just a sequencing tool. It has huge benefits in terms of giving you a template for your project which you can use as the basis for cost calculations. Once your CPA is complete, your quantity surveyor or estimating team can use it to cost each task in the project and create an overall project budget.

Cost Savings – When your project budget is available, your team will be able to look for possible cost savings. This may be achieved by getting certain materials or labour to site earlier that first estimated. If you are using contractors, it may give you the insight to be able to negotiate harder with them, knowing when you can push them to complete or to identify the stages where you might want to increase resources in order to speed up a section of the project which lagged last time.

Works Well with Other Tools – Your CPA will work well alongside other project management tools that you may already be using. It can be used very successfully to compliment other tools such as Gantt charts and Programme Evaluation Review Technique (PERT).

Drawbacks of Using CPA

Learning Curve – If you haven’t used CPA before, there will be a learning curve that you will need to work through in order to get things set up. Specialist software such as MS Project will do a lot of the legwork for you, but you will need to learn how to use it in order to set up and update your CPA.

Daunting Set Up – The idea of specifying all of the tasks in your project in granular detail can be a daunting prospect. It can be a lengthy and time consuming exercise to begin with, especially if you are relying on colleagues, contractors and other third parties to provide the information required to populate the model. However, CPA is fundamentally a simple process and once set up, you will have the data as starting point for future projects. Fundamentally, the benefits of CPA far outweigh the effort required.

Constant Updates – Once you have set up your CPA model, you will have a snapshot of your desired path to achieve your project. But that’s only the beginning. To use it in anger and realise its power, your CPA should become a living document that is continually updated throughout your project. So, you will need to have the resources and discipline to do so.

Using CPA in Your Project

CPA gives you the tools to adjust future project programmes to ensure that your scheduling should becoming more and more accurate.

As your project gets underway and you update and revise your critical path, you may find ways to improve the work process or accelerate the project. Sometimes it will be necessary to incorporate changes and additional tasks which were not part of the original critical path.

This ongoing process will give you a blueprint for your project, which can then be used as a template for subsequent ones. Returning to our housing development example, data from their CPA may help the developer decide to procure more materials in advance in future, create a better programme for moving contractors on site from one house to the next and improve how their initial estimates perform against actual delivery.


CPA is powerful project management tool for construction projects because it enables you to focus your attention on those tasks which are critical to the project timeline.

It provides a straightforward method to help you to make more efficient use of resources, identify where project bottlenecks may occur, and ultimately gives you tools to decide how project timeframes can be reduced and cost savings achieved.

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About Karen Toms

Karen Toms is an experienced project manager who has been managing commercial construction projects for the past eight years. She has also worked as a marketing consultant with a variety of property and construction companies.

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