S Curve in Project Management

S-curve is a graphical display of cumulative data that is plotted against time. And that cumulative data here can be anything, such as cost, man-hours, quantity progress, or different types of project information that you want to plot against time. And don't forget, an S-curve is the cumulative plot of unit data that you plot against time. You may find S-curves with other names; you might hear it called an S-plot, or another name could be cumulative distribution chart, or velocity diagram, or in an earned value management environment, they call it a performance measurement baseline or PMB. So, don't be surprised if you hear it called other things in projects that you have been working on. But the bottom line is you're drawing an S-curve in projects.

Now, why the name 'S-curve'? Is it in the shape of an S? The answer is yes. A scale forms the shape of an S in most projects. So, the S shape, the shallow S, is a typical shape of a cumulative graph that you will see in projects. And the reason is, if I just plot the incremental data in a project for you so you understand why the cumulative data shows like an S-curve, it's because in most projects, you start very slowly in the project, and then you ramp up during project execution, and then you ramp down as you close out the project and deliver most of the deliverables, basically finishing the project. So, you will see that as a typical shape in most projects. If I convert this to a cumulative curve, then that would be something like a S-curve that you see here, with the orange bar. So, that's why it creates an S shape for you based on the essence of the projects. But having said that, don't be surprised if your S-curve, the cumulative curve that you draw in a project, does not look like an S shape. The shape of an S-curve depends on the nature of your project, what kind of contract you are working on, and depending on the project, you might see different types of S-curves.

Types of S-Curves

One of the most popular ones that you will see is a front-loaded S-curve. And you can see here in this graph, most resources are consumed early on in the project, and then you ramp down very quickly toward the end, and nearly most of the load is at the beginning of the project. Examples of this would be projects that are accelerated early on, so something happens and you're just accelerating that project quickly early on. You put in a lot of resources at the beginning of the project to finish the project based on the contract requirements. So, then you will end up with a curve like a half dome rather than an S-shape because you have a front-loaded plan in your project. Another example would be for repetitive projects, something that you have done over and over again. So, you are quite familiar with that, so you don't need that much of a learning curve at the beginning of the project. So, you know what to do, so you just put resources earlier on and finish the job as quickly as you can. So, that's another example of front-loaded plans that you will see in a project.

Another example would be for an urgent quick repair of damage early on in the project that you need to have resources to repair that damage. So, you will put a lot of resources early on to repair that. So, that would be another example of a front-loaded S-curve. And if you are drawing cost information, the mobilization cost or the deposits that you put for some procurement items, then that would have the tendency of creating an S shape for your cash flow curve as well. So, these are some examples of the projects that you may see forming an S-front-loaded S-curve. But having said that, make sure that if you end up with a front-loaded S-curve, evaluate first of all whether you have resources available early on in the project to execute your project like this, and second, is your plan valid? Is this the project that you are really putting a lot of resources early on in the project execution? So, you have to validate your plan and validate the available resources that they're going to execute your project. So, make sure you validate this before accepting this S-curve as your plan for the project.

Another form of S-curve that you will see, which is completely opposite of this curve, is a back-loaded S-curve. So, as you can see, this one starts very slowly here on the project and then ramps up toward the end of the project. And examples of this type of back-loaded S-curves would be complex projects, projects that you need a lot of planning and design early on, so it takes a lot of learning curve, basically, before you jump into execution and start executing the job. So, that would be an example of a back-loaded S-curve that you may face. Just like front-loaded S-curves, make sure that you validate the basis of your plan and availability of your resources. And again, you have to question the back-loaded or front-loaded S-curves in projects to make sure your plan is right.

To give you a perspective of these three shapes, I put them all in one graph for you so you can clearly understand the difference between the front-loaded, back-loaded, and a typical S-curve. And here is the curve of three of them in one graph. As you can see, that's the front-loaded S-curve, kind of a very sharp curve, as I'd like to call it a half-dome curve. This is the back-loaded curve; again, it just starts very slowly and ramps up toward the end. But the green one is the typical S-curve, kind of a shallow S-type curve that you will see in most projects. So again, make sure that you validate your plan for both front-loaded and back-loaded curves before you accept them as a baseline or a plan moving forward for comparison.

So, these were different shapes of S-curves that you will see in a project depending on the nature of your project. Now, the question might be, what kind of information should I plot in an S-curve? First of all, the x-axis would always show your time, and the time intervals would be based on your project reporting and updating cycle. So, if you are doing your updating project reporting every month, then it would be on a monthly basis, or it can be bi-weekly or weekly. So, the shorter these time intervals, the smoother your S-curve would be and the easier for analyzing the trends in your project. The y-axis always shows cumulative data, and the cumulative data can be different data that you want to show and plot in a project. It can be man-hours, cost, FTE, progress, quantity, resources, or different types of information that you want to analyze in a project. But among all of those, the three most common types of S-curves that you will see in projects are these three.

You may see a cost S-curve, which is basically your cost versus time, and this curve is very popular for cash flow projections, kind of showing the spending of your budget over time or cash flow curve. Another popular S-curve that you will use in projects is a man-hour or FTE (full-time equivalent) curve, basically showing the available resources that you have over time, either the headcounts or man-hours. Again, that's very popular; you will see that a lot as well. Another type of S-curve that you will see in projects is a quantity S-curve, so here would be a quantity of materials, equipment, or something that you want to measure against time. And that's a very good chart for comparing what you were planning in terms of quantity and what you actually accomplished, like installing equipment or something. So, that would be a very good comparison chart that you will use in projects.

It is generally used in;

See also: 

Schedule Forecast