A S-curve, named due to the S form shaped shape of a curve, is a management tool that depicts a mathematical graph of relevant cumulative data over time.
S Curves are widely used in projects, as well as in production organizations to record, monitor, analyze and control various performances by measuring parameters over time. They provide a quick overview of projects’ current status, comparisons between the baseline or targets, as well as the forecast for future periods.
For the projects of significantly longer time frame such as infrastructure development or construction of an industrial plant, the data is populated every week or on monthly basis, whereas for smaller projects where each day is critical to monitor such as in plant turnarounds and shutdowns, this graph is updated every day to reflect the current as well as the historical status of the project.
Inputs for S-Curve
S Curves are developed using a monitoring parameter and a time interval.
Monitoring parameters are the numbers that need to be updated, monitored, presented, analyzed, controlled, and forecasted such as Cost, Progress, Labor Units (Man hours), number of deliverables, and so on.
The time interval is the frequency at which these parameters need to be monitored and are normally expressed in days, weeks or months.
Monitoring parameters are normally plotted on vertical axis while time on horizontal.
What is the S-Curve Used for?
In today’s advanced business climate, ensuring the project is on schedule and under or on budget is paramount to its success and S Curves are widely used for this purpose.
Here are some of the common uses of S Curves in projects management organizations;
1 - Schedule Analysis & Leveling:
During the early stages of a project, when planning processes are happening, this graph is used to finalize the baseline schedule.
Data (mainly in the form of man hours and cost) is taken from the resource loaded schedule and the S Curve is plotted to review and analyze the planned performance.
Any ups and downs as highlighted in the S Curve are then leveled and adjusted back into the schedule using various techniques (such as fast tracking or crashing) to come to a uniform and realistic S Shaped trend.
This is an important step during schedule development as any unexplained peaks and downs can result in irregular mobilization and demobilization of resources which is not viable and practical as well as put a massive financial burden on the project budget.
A uniform S Curve generated from the resource leveled schedule is also an indication of optimum utilization of resources and cash flows.
2 - Performance Evaluations & Comparisons
S Curves are also used for performance evaluations and comparison purposes: Comparison of actual and planned parameters.
During the planning phase of project cycle, the project schedule is developed and a baseline is frozen. Resultantly all the monitoring parameters are plotted on the basis of baseline schedule in the form of S Curve.
This is the initial curve stated as planned curve.
As the actual work is performed during the execution phase of project, all the actual data is reported at a set frequency and plotted on a relevant S Curve as the actual one.
The curve then clearly indicates any positive or negative variance of actual and what was planned during the planning phase and this comparison allows the projects managers and team members to see the overall status of parameter and further decision makings.
Some of the parameters which are mainly generated and monitored with the S Curves are listed below;
- Planned Progress Vs Actual Progress
- Planned Man hours Vs Actual Man hours
- Planned Cost Vs Actual Cost
- Planned Quantity Vs Actual Quantity
Progress, Man hours, and Cost are normally plotted to see the overall status of the project whereas the quantities are plotted at individual discipline level to see the details of each discipline contributing to the overall status of the project.
Examples of quantities that are normally monitored in the form of S Curve at construction project are listed as below;
- Engineering deliverables
- Concrete pouring
- Steel installations
- Equipment installation
- Piping installation
- Cable laying
- Cable terminations
3 - Forecasting
Another use of S Curve is found in forecasting purposes.
As the project continues and the S-curve grows with the data populated at a regular frequency, the graph will turn into a historical representation.
This is used to predict future trends based on historical performance.
Progress and cash flow forecasting and resource mobilization or demobilization planning is important in this context based on work front availability and historical performance to have a clear picture of future status and requirement for project and thus taking any preventive control measures before it gets worse.
Interpretation of S Curve
Interpretation of S Curve is important for the team members as it helps to identify the actual performance of any monitoring parameter with the set target (baseline).
As the S Curve is plotted with the cumulative numbers, if the actual curve is above the planned curve, it is a positive variance meaning the actual performance is higher than planned one and the project is ahead of baseline schedule.
While on the other hand, if the actual curve is progressing lower than the planned one i.e. negative variance, this is a sign of further analyzing the performance and looking into more details of work and taking necessary corrective actions to bring the project on track.
As the project progresses, there can be ups and downs in the actual performance compared with the planned one. As the S Curve is the reflection of what is being done actually, this data must be communicated with the project manager and the discipline team members to put the timely efforts to maintain or to improve the project performance.
The pictures below illustrate a projects’ typical progress S Curve. Figure-1 can be seen where actual progress line is above the planned curve with a positive variance of 7.0% (Ahead of Schedule) while Figure-II shows the actual progress curve below the planned curve with the negative variance of 5.0% (Behind Schedule).
S Curve Development Tools
S-Curve can be generated in Primavera P6 from the resource loaded schedule as well as the schedule resource data can be taken out and S Curve could be generated in Microsoft Excel for better and customized graphical formatting.
ScheduleReader is also an important tool in this context and can be used to present primavera S Curves in a better and presentable format for team members as well as executive management.
The software is capable of showing the S-curves for the following data: units and costs.
To apply the s-curves into Resource Usage Profile view, do the following steps:
- In the Activities Format menu, select the Options feature from Resource Usage Profile group.
- Select any check-box from the Cumulative column (from example: Budget or Actual).
You will get a plot of the S-curve as shown in the image below.
Types of S-Curves in P6
In Primavera P6, S Curve is plotted for several parameters against the time interval to see the comparison.
These parameters are listed below;
- Labor Units
- Non-Labor Units
- Labor Cost
- Non-Labor Cost
- Material Cost
From the above list, units can be plotted either labor or non-labor at one time however cost can be plotted as individual or the total of all costs associated with the activities.
Moreover from the above list of parameters, various types of S Curves could be generated in Primavera P6 which are listed below and can be chosen by the project manager and team members to reflect the appropriate one at any given time.
- Baseline Curve
- Budgeted Curve
- Actual Curve
- Remaining Early Curve
- Remaining Late Curve
In addition to the above types, Earned value curves can also be reflected in Primavera P6 which are used for earned value analysis of the project and are mentioned below;
- Planned Value Cost Curve
- Earned Value Cost Curve
- Estimate at Completion Cost Curve
S Curve is an important and widely used management tool across project management and project control.
It helps the project team and project controls professionals monitor the various forms of performance outputs, identify any cracks and take preventive and corrective control measures to ensure on schedule and under budget completion of projects.
The S Curves of completed projects also become part of the lesson learned document for organizations as guidance for future similar projects.