S Curves: An Introduction

2006.08.31 | Bob Galen | Software Development

Project managers often track project schedules and milestones as a mechanism for managing the project and detecting risk. The monitoring is usually at a task level of detail and often quite labor intensive. As it turns out, there is another mechanism for monitoring project state. While it�s not at the finely grained level of detail as the individual task, it does provide wonderful insight into the nuance of a project.

You see projects have a tempo to their development that is usually consistent over time and that can be represented graphically by plotting cumulative effort vs. time. The tempo usually takes the form of an S Curve which represents the normal ramping of a project.

S Curves Defined
An S Curve is a graphical display of cumulative cost, value, labor hours or in our case Active Time vs. time. The name derives from the S like shape of the curve, flatter at the beginning and end and steeper in the middle, which is typical of most software projects. The beginning represents the typical slow, methodical start to projects, as the team sorts through the work and determines how to approach the project. The end represents a deceleration as the work runs out and the middle is the acceleration period as the team attacks the work.

Other Terminology
Often S Curves are used to determine project variance according to a plan. In this case, a Baseline S Curve is established from the initial project plan, for example for hours. As the project continues, natural changes occur. These are then mapped as the Target S Curve, which represents all new, approved work beyond the initial baseline. Finally, the Actual S Curve is graphed cumulatively in real-time for the project. The comparative differences between the 3 curves, Baseline, Target, and Actual can shed interesting details on progress and challenges within your project.

Figure 1, provides an example of these curves. In this example, the curves illustrate growth in Baseline to Target�in other words, expansion of the originally planned scope.

figure 1

Tracking Progress
While analyzing Baseline S Curves can be interesting, the real value comes from comparing Actual against your expected Target S Curves. Target in this case can be assumed to be your planned level of progress.

Comparison of the Target S Curve and Actual S Curve reveals the relative progress of the project over time. In most cases, the Actual S Curve will sit below the Target S Curve for the majority of the project. Towards the end of the project the curves should converge and meet. If the project is ahead of schedule, the Actual S Curve will rise above the Target S Curve. However, the Actual S Curve can never finish above the Target S Curve.

figure 2