THE MYSTERIOUS S CURVE

THE MYSTERIOUS S CURVE
By Greg Gendall
INTRODUCTION
The first time most Project Managers become aware of the existence of S Curves is when they are requested by the Client or senior management to include one in their next progress report.
This article explains what the mysterious S Curve is, why it is an important project management tool, and how to generate one.
WHAT IS A S CURVE?
A S Curve is defined as "a display of cumulative costs, labour hours or other quantities plotted against 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 projects. The beginning represents a slow, deliberate but accelerating start, while the end represents a deceleration as the work runs out."
(Source: Wideman Comparative Glossary of Common Project Management Terms v2.1 Copyright R. Max Wideman, May 2001)
TYPES OF S CURVES
There are a variety of S Curves that are applicable to project management applications, including...
1. Man Hours versus Time S Curve
2. Cost versus Time S Curve
3. Baseline S Curve
4. Actual S Curve
5. Target S Curve
6. Value and Percentage S Curves
MAN HOURS VERSUS TIME S CURVE
The Man Hours versus Time S Curve is appropriate for projects that are labour intensive. It shows cumulative man hours expended over time for the duration of the project. As man hours are a product of man power and working hours, these may be adjusted individually or together to ensure the project remains on schedule.
COST VERSUS TIME S CURVE
The Cost versus Time S Curve is appropriate for projects that contain labour and non-labour (e.g. material supply / hire) tasks. It shows cumulative costs expended over time for the duration of the project, and may be used to assist in the calculation of the project's cash flow.
BASELINE S CURVE
Prior to project commencement, a schedule is prepared outlining the proposed allocation of resources and the timing of tasks necessary to complete the project within a set time frame and budget. This original schedule is referred to as the Baseline Schedule. From this schedule, a Baseline S Curve is generated. This S Curve reflects the planned progress of the project.

Figure 1: Baseline S Curve
TARGET S CURVE
Following project commencement, modification of the Baseline Schedule is usually required. Changes are continually made to the Production Schedule (which is originally the same as the Baseline Schedule). The production schedule reflects the actual progress of the project to date, and any revisions made to tasks yet to commence or not yet completed. From this schedule, a Target S Curve may be generated. This S Curve reflects the ideal progress of the project if all tasks are completed as currently scheduled...

Figure 2: Target S Curve
ACTUAL S CURVE
The production schedule is updated on a regular basis throughout the duration of the project. These updates include the revision of percentage complete for each task to date. Using this information, an Actual S Curve may be generated. This S Curve reflects the actual progress of the project to date, and may be compared with the Baseline and Target S Curves to determine how the project is progressing.

Figure 3: Actual S Curve
VALUE AND PERCENTAGE S CURVES
S Curves may be graphed as absolute values (i.e. Man Hours or Costs) versus Time, or as percentage values versus Time. Value S Curves are useful for determining Man Hours or Costs expended to date, and Man Hours or Costs to complete. Percentage S Curves are useful for calculating the project's actual percentage complete against target and baseline percentage complete, and for calculating the project's percentage growth (or shrinkage).
WHY USE A S CURVE?
S Curves are an important project management tool. They allow the progress of a project to be tracked visually over time, and form a historical record of what has happened to date. Analysis of S Curves allow project managers to quickly identify project growth, slippage, and potential problems that could aversely impact the project if no remedial action is taken.
DETERMINING GROWTH
Comparison of the Baseline and Target S Curves quickly reveals if the project has grown (Target S Curve finishes above Baseline S Curve) or shrunk (Target S Curve finishes below Baseline S Curve) in scope. A change in the project's scopes implies a re-allocation of resources (increase or decrease), and the very possible requirement to raise contract variations. If the resources are fixed, then the duration of the project will increase (growth) or decrease (shrinkage), possibly leading to the need to submit an extension of time claim.

Figure 4: Calculating Project Growth using S Curves
DETERMINING SLIPPAGE
Slippage is defined as "the amount of time a task has been delayed from its original baseline schedule. The slippage is the difference between the scheduled start or finish date for a task and the baseline start or finish date. Slippage can occur when a baseline plan is set and the actual dates subsequently entered for tasks are later than the baseline dates or the actual durations are longer than the baseline schedule durations." (Source: Wideman Comparative Glossary of Common Project Management Terms v2.1 Copyright R. Max Wideman, May 2001)
Comparison of the Baseline S Curve and Target S Curve quickly reveals any project slippage (ie the Target S Curve finishes after the Baseline S Curve). Additional resources will need to be allocated or additional hours worked in order to eliminate (or at least reduce) the slippage. An extension of time claim may need to be submitted if the slippage can not be eliminated or reduced to an acceptable level.

Figure 5: Calculating Project Slippage using S Curves
DETERMINING PROGRESS
Comparison of the Target S Curve and Actual S Curve reveals the 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 (due to many factors, including delays in updating the production schedule). Only towards the end of the project will the curves converge and finally 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 6: Determining Project Progress using S Curves
PROJECT BENCHMARKS
Percentage S Curves may be used to calculate important project benchmarks on an on-going basis, including:
1. Project percentage growth (Target and Baseline S Curves)
2. Project percentage slippage (Target and Baseline S Curves)
3. Actual percentage complete against Target percentage complete to date
4. Actual percentage complete against Baseline percentage complete to date
HOW IS A S CURVE GENERATED?
In order to generate a S Curve, a Baseline and Production Schedule are required. These schedules should contain the following information for each task...
1. Baseline / Actual Start Date, Finish Date
2. Baseline / Actual Man Hours, Costs
3. Actual Percentage Complete
Some software scheduling packages automatically generate S Curves. On the other hand, some (including MS Project) do not. In this case, a third party software application is required to process the Baseline and Production Schedule data to generate the required S Curves.
Midori Media's myPM SCG: S Curve Generator is a standalone MS Windows application that integrates with MS Excel to generate the various types of S Curves discussed above. MS Project users will need to export their project data to a MS Excel file (easily accomplished using the Export file option). myPM SCG processes the resulting export file, and creates a MS Excel output file containing the required S Curves. These may be pasted to MS Word for inclusion in progress reports.

Figure 7: Generating S Curves
2003 © Greg Gendall