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 (e.g. change of scope, delayed start), the Baseline Schedule may require revision to reflect the changed requirements.
Figure 3: Baseline 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
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 terminate at the Cut Off Date. This is the date the Production Schedule was last updated. At the completion of the project, the Actual S-curve will meet the Target S-curve.
Figure 5: 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 contraction).