S-Curve is a graphical representation of the financial flow of a project. It is called an S-Curve because the cumulative curve typically mimics or appears like the letter “S”. An S-Curve is essential in project management because it helps visualize the progress trend, compare actual progress with planned progress, and predict cash flow.
S-Curves are mainly prepared based on non-unit resources in Primavera P6, i.e., cost or cash. All activities are assigned these non-unit resources, and based on the schedule and resource loading of each activity, P6 generates the planned cost over time, which can be used to plot the S-Curve.
Let’s assume a project schedule with the activities, schedule, and BOQ values as presented in the table below. For each activity, the planned amount for each day can be calculated using the formula:
Activity BOQ Value / Activity Duration
The amounts planned for all activities on a particular day are then summed together, which provides the planned cash flow on a daily basis. For an S-Curve, this daily cash flow needs to be accumulated. For example, Day 1 will show the amount planned up to Day 1, while Day 2 will include the amount from Day 1 + Day 2. Similarly, the cumulative value on the last day will include the planned amount from Day 1 through the last day. This cumulative planned cash flow is then plotted against the date, producing the planned S-Curve.
Similarly, based on the actual work progress at site, each activity generates a value based on its progress percentage multiplied by its BOQ value. The actual value generated for all activities is calculated for each day, providing the actual cash flow. Therefore, the actual cash flow can only be determined up to the current date. This is referred to as the Budgeted Cost of Work Performed (BCWP), also commonly known as Earned Value (EV).
By comparing the cumulative planned value up to the same date, known as the Budgeted Cost of Work Scheduled (BCWS) or Planned Value (PV), the Schedule Performance Index (SPI) can be calculated:
SPI = BCWP / BCWS
If SPI > 1, the project is ahead of schedule, while if SPI < 1, the project is behind schedule. This performance can also be easily visualized on the S-Curve by comparing the actual progress curve with the planned curve.
Thus, overall, the S-Curve is a powerful tool for tracking project progress at a higher level. Every weekly project meeting should present the S-Curve to provide a clear overview of the current project status and identify whether the project is progressing as planned.
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