The S-Curves in Project Management - Guide article provides a very good breakdown of how and the reasons for creating an S curve.

What is an S-Curve in Project Management?

Project managers are responsible to exert control over the project to ensure the cost and schedule are met and not exceeded. The S-curve a time phase expenditure for the project, both in time (dollar amount) as well as any capital expenditures.  

This connection is via the work breakdown structure (WBS), and used to track actual performance against the plan to assess the actual time and cost against the planned time and cost.

This article is a picture-perfect illustration of how to use this technique. Text book perfect.

However, there are some things we can add to this perfection. As always, managing a project, monitoring and controlling, is not as simple as this simple math.

S-Curves Prerequisites

S-Curves don’t just happen.

There is plenty of information from estimates to reporting systems for time and investments. Parameters and time frames need to be set up initially, specifically the effort and what we plan to spend over the time of the project. This will require time reporting and accounting systems to be in place. In building construction, each discipline or division is given a specific account number attached to the project number, either as a prefix or suffix. This allows for better tracking of each division and makes it easier to track actuals.

A few of the monitoring parameters to consider are cost, deliverables, person-hours, juxtaposed against the schedule.

Here is another article that further talks about the tracking benefits of S-curves.

The time intervals will depend on length of project. One should also consider the comfort level of oneself and the stakeholders, if the latest S-Curve provides negative information.

For example, the project planned expenses vs actual expenses was planned to be reviewed on a monthly basis. One monthly S-Curve shows very high or very low actuals, a decision to review on a weekly basis until actuals are back on-track.

After putting these items in place we must have a great deal of discipline in the use of those systems and techniques. Essentially, we need a way of estimating, and time phasing the expenditures, you will see that demonstrated well in that article.

We need this to be time phased.  After decades of project management experiences, these prerequisites are seldom entirely understood, and even worse, met.

To articulate the difficulties, we break the process challenges down into the Road Map Side, that is the planning effort, and then The Recording Side, which is the actual work effort.

The Road Map Side

Work Breakdown Structure

To be able to create the curve and make any benefit from it requires some up-front work that is foundational for project management, and that is the Work Breakdown Structure (WBS).  To build the time phase budget, which essentially is the S curve, or the curve (more on that later).


We are going to need estimates, ideally connected to the breakdown of the effort. We are certain most of you out there already know the challenges with accurate estimate generation. It is not easy, especially when the things we are trying to estimate are things with which we have little experience. It suffices to say that the quality of the estimates will have a HUGE impact on the quality of the curve and project cost.

We will need to connect the estimates to the budgets for specific work element in the WBS. This is noted in the article we pointed to.

However, it is critical for the project manager and the organization at large, to understand the curve is pointless, meaningless, if we do not connect the dots starting with the budgets for the work and time phased. By time phased, we mean that the expenditures are connected to some work over some specific period of time, more importantly duration. From experience this does not always happen, or, more like seldom.


To make this work, as explained in the article treating the mechanics of the S-curve, the estimates are connected to the WBS. This will require a clear scope of each of the individual efforts as defined in the WBS.  The details of the scope will be defined in the WBS dictionary.  The connection to the accounts, reporting mechanism (time and capital spending), and the individuals performing the work is necessary.  The team members working on specific WBS elements will need to know the specific WBS and record their efforts in the service of these objectives.

The Recording Side

Time and spending reporting systems

From experience, the time and cost reporting systems are not always effectively connected to the project and those executing the project.  We will need some mechanism to connect the estimates for the work connected to date range of the work to be accomplished along with the costs associated.  To do this, will require a suitable level of disaggregation (small enough breakdown) of the project efforts to produce the desired results.

Then we will need our team members reporting connected to those specific accounts ideally associated with the WBS elements. By way of example, I am working on a deliverable associated with WBS element PRJ22-1000-12.  I will need to book my hours to this element to achieving the objective of this element.  If I apply my effort to some other element, or a higher order element, we have no traceability to this specific effort.  

From experience, this is significant source of failure.

Diligent use (discipline)

If systems capability is source of failure, then actual reporting is an even greater risk.  My brother, Shawn Quigley, has a position where he tracks the actual performance against the planned, and from his experience it is nigh impossible to get the team members to consistently repot their effort against the specific WBS work element that applies, rendering the benefits of the S-Curve and tracking impossible and likely a wasted effort.

Missing or unused account number

Accounts that are not included, or not connected to those performing that part of the work will result in a curve that does not create the curve from which we can understand and take appropriate action. Missing accounts will result in a similar failure mode as lack of diligent use of reporting either the hours (cost) or capital spending.

Insufficient breakdown and delineation of effort

The way a curve appears to us, will depend greatly on the resolution of the data that creates the curve.  

Long periods before sampling will impact the shape of the curve, and the conclusions drawn. Our ability to respond appropriately is predicated upon the data and our interpretation of that data, and the ability to invoke specific controlling actions. That is the reason for taking the time and effort, the ability to understand and exert control.

s-curves in project management

No metrics or errant metrics

The amount of time we have spent to achieve the objectives associated with the WBS.  

For example, we have estimates of durations (cost) coupled with capital investments for things like tooling.  Let’s say that we have 200 hours estimated to for a specific set WBS deliverable. Those 200 hours are used over specific time, for example to produce 4 requirements documents. To connect the metrics to the time, we would say that accomplishment of 1 of the documents should take 50 hours.  

The effort and the result need to be connected to be able to benefit from the curve and the ideally overlay performance curve.  

There are a number of failures associated with metrics, in general these errors fit 3 categories. We note these here, and if there is interest, we can create an article that covers these in detail.

  • Systematic error
  • Random error
  • Human error
  • Tracking of spending at specific time

Tracking the progress of the spending on a time interval that make sense of the project will be important.

Is an S-curve in project management always an S shape?

No, an S-Curve graph is not always a nice-looking S. The quality of the data being input to create the curve is very important.  

The plan for the project spending, for example the duration of the project and how the work effort and the spending occur in that time.  We should not believe that every project type and duration will result in an S shaped curve.  

Some examples:

  • Incomplete data
  • Incomplete data of one component
  • Time periods don’t line up
  • Time charged against a project may lag expenditure reports
  • Expenses reports lag
  • Projected expenditures report but not actuals to date