Critical Path Method (CPM) In Project Management

The point of the critical path method in project management is easily worked out from the name.

It seeks to find a path through a project based on the critical tasks that need to be completed and prioritises these.

Imagine if you have hundreds of tasks planned out and each of these have many dependencies. Prioritising is almost impossible but essential if the project is to be efficient as well as effective.

The critical path method helps to identify these important tasks.

The critical path is the longest sequence of tasks in your network diagram that must be completed if the project is to be delivered on time.

It can also be defined as the shortest amount of time needed to complete the project. This is essentially the same thing.

If you are building two houses and one will take 14 months to complete and the second house 6 months to complete, you can wait 8 months to start the second house but still the whole project will not be complete for 14 months. Building the first house is the critical path and the shortest duration the project can be completed.

If there is a delay on this critical path then the project will be delayed.

The flow diagram of critical work tasks is used to estimate the duration of the whole project by estimating how long each of these individual tasks will take.

The selection of these individual tasks is determined by how crucial it is that they are completed on time. If there is no pressure of time on a task then this is not part of the critical path and can be pushed back.

The Basic Principals of Critical Path Method

Imagine you are making a roast lunch.

The length of time it takes to cook the meat is a critical task, as well as how long it needs to rest before it can be served. You would want to make the gravy last and have the vegetables steaming along the way.

There are other tasks that are not time critical – warming the plates, for instance or setting the table – and these can be fit in where there is a gap in the process.

So, the time it takes to cook a roast lunch will depend on preparation time of the vegetables, roasting the meat, resting the meat, producing the gravy.

The entirety of this critical path will dictate how long the whole task will take. If you are delayed in putting the meat into the oven then the whole meal will be delayed.

First you draw a network diagram, where you draw out the different activities and how they are connected to each other.

As you draw out this network diagram, with each task reflecting a separate box flowing into another, you identify all the paths through the project. A project is where one task follows another task follows another task.

As you draw these out you hope that there is only one critical path – though there is a possibility of two or more critical paths that you are going to have to manage in parallel – and there is no float (time to spare) in any of these paths – so you cannot afford a delay.

If you face a delay then you can fast track or schedule crash to bring the project back on track.

After you have worked out the critical path or paths, you can then calculate the float in the other paths that allows you to work out how far you can push them back, if necessary.

From this point you need to work out the early start, early finish, late start and late finish.

How Does Critical Path Method Work?


Source: ActiveLabs

There are lots of words and phrases here that may appear confusing. It is well worth breaking down the process into steps:

#1 A network diagram is a flow diagram of all the tasks that need to be done and where one job is reliant on another being finished these are put in order. The number of days or weeks it will take to complete the task is written in the box.

#2 Add all the boxes together for each path through the network diagram – with the largest number of days and weeks being the critical path.

#3 Work out how much float there is for each of the other paths. This number is the number of days it will take to complete the critical path minus the number of days it will take to complete the shorter path.

#4 Calculate the early start and early finish of the critical path. To do this you start at the beginning of the path and work to the end. The early start for the first activity will always be 1 and the early finish depends on how long it will take. Should it be 10 days then the early finish of the first activity will be 10. The early start of activity 2 will then be day 11. The early start will be day 11 plus the number of days it takes to complete the activity. And so on through the critical path.

#5 Calculate the early start and finish of the other paths. This is the same as calculating the start and finish of the critical path except you must take account of there being different dependencies. For instance, a task may not be able to start until two different previous activities are complete. The start date of the next activity is the longest dependency plus one day.

#6 Calculate the late start and the late finish. This means considering the latest date you can start a task on non-critical paths and still the project would be complete. So, if there is a float of 14 days in one path then along that path these 14 days can be programmed in.

If you find your schedule slipping for the critical path, you have two choices: fast-tracking and crashing.

Fast tracking is when sequential activities are performed in parallel if possible. You should identify the potential for fast tracking activities when drawing out your network diagram. It might be that you can only run these activities partially in parallel but still this saves time without spending more money.

Crashing is a compression technique where you bring in extra resources to speed up the schedule. You review the critical path and see which activities would be completed quicker if more resources were used – this might mean giving overtime, buying more resources or offering rewards for harder work. This won’t always work to speed up the schedule and it will increase the cost of the project.

The network diagram with all the days mapped out around the boxes is a useful graphical view of the project – and thankfully there is software that will calculate all the starts and finishes and the float days – including the free floats.

It is an excellent means of making the dependencies visible and helps with scheduling, and if necessary, rescheduling of work. It helps set up contingencies and step in if a deadlines drift and a project manager needs to get it back on track.

The problem with finding out you have these days spare on some paths makes it easy for tasks and activities to drift.

It also makes some activities more important than others, with the possibility of non-critical paths not receiving the attention they deserve.

They are only on the critical path because of time and not because of value to end product and this can sometimes be lost in the melee of managing days in the project.

It is important to consider constraints with resource availability – and this can sometimes be missed in critical path method.

In Summary


Source: ActiveLabs

This is a complicated method but one that helps the project manager to manage schedules.

Where a project is guided by a strict deadline, then this is an excellent method. During the project the manager will spend a lot of time keeping the critical path on time in under to bring it in to deadline.

However, it is also the role of the project manager to keep an eye on the network diagram – making sure that other paths do not slip and suddenly become a critical path in themselves.

The maths might seem complicated but it is about considering how days build on days.

There are also software programs now that help you to lay out the tasks, put them into sequence and consider the start and end dates for each task in the project.

This is useful because you have a clear set of days to tell the team, which allows them to see clearly when they will be needed and when there are time critical moments in the flow of work.

If you are one of those people whose mind works in a series of flow diagrams, then this is an excellent way of managing a project.

However, if you find Critical Path Method unsuitable for your organisation, check our guide to some of the most popular Project Management Methodologies.

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