Why Use an 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. Analyses of S-curves allow project managers to quickly identify project growth, slippage, and potential problems that could adversely impact the project if no remedial action is taken.
Comparison of the Baseline and Target S-curves quickly reveals if the project has grown (Target S-curve finishes above Baseline S-curve) or contracted (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 (finish later) or decrease (finish earlier), possibly leading to the need to submit an extension of time claim.
Figure 6: Calculating Project Growth using S-curves
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".
Comparison of the Baseline S-curve and Target S-curve quickly reveals any project slippage (i.e. the Target S-curve finishes to the right of 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 cannot be eliminated or reduced to an acceptable level.
Figure 7: Calculating Project Slippage using S-curves
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. The Actual S-curve can never finish above the Target S-curve. If the Actual S-curve sits above the Target S-curve at the Cut Off Date, the Production Schedule should be examined to determine if the project is truly ahead of schedule, or if the Production Schedule contains unrealistic percentage complete values for ongoing tasks.
Figure 8: Calculating Project Progress using S-curves