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. The following 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... * Man Hours versus Time S Curve
* Costs versus Time S Curve
* Baseline S Curve
* Actual S Curve
* Target S Curve
* 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 together or individually in an attempt to keep the project on schedule. Projects may require additional man hours to finish on time due to low productivity, delays and disruptions, rework, variations, etc.
Figure 1: Man Hours versus Time S Curve
Costs versus Time S Curve
The Costs versus Time S Curve is appropriate for projects that contain labour and non-labour (e.g. material supply / hire / subcontract) 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 cashflow, and cost to complete.
Figure 2: Costs versus Time S Curve
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 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. If the project requirements change prior to commencement (eg. change of scope, delayed start), the Baseline Schedule may require revision to reflect the changed requirements.
Figure 3: 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. In an ideal world, the Target S Curve will meet the Baseline S Curve at the end of the project (On Time, On Budget) or finish below and to the left of the Baseline S Curve (Early, Under Budget). In reality, it is not uncommon for the Target S Curve to finish above and to the right of the Baseline S Curve (Late, Over Budget).
Figure 4: 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. During the project, the Actual S Curve will...
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