S curve

An S curve plots time along the horizontal axis and a cumulative measure on the vertical axis. That measure might be cost, earned value, labour hours or percentage complete.

The shape follows the natural rhythm of a construction project. Early months are slow: site establishment, demolition, piling and footings. Spending then accelerates as structure, services and fit-out run in parallel with many trades on site. Towards the end, the remaining work is finishes, commissioning and defects, so the curve flattens out.

The planned S curve is usually built by loading the cost plan or contract sum onto the construction programme, so each activity's value is spread over its planned duration and added up month by month.

The value of an S curve comes from comparing lines on the same chart:

• Planned or baseline curve. Expected cumulative value or cost if the project runs to programme.

• Actual cost curve. What has really been spent to date.

• Earned value or claimed curve. The value of work actually completed, often taken from progress claims.

• Forecast curve. The expected path from today to completion.

If the earned value line falls below the planned line, the project is behind. If actual cost runs above earned value, the work is costing more than planned. Clients and financiers often ask for an S curve with each progress claim to check that claims match physical progress.

S curves are also used for cash flow. A developer or principal plots the expected cumulative payments to the contractor to plan funding drawdowns, while the contractor plots expected income against expected outgoings to subcontractors to check the project stays cash positive.

A Gantt chart shows individual activities as bars against time, with their start, finish and links. It answers what is happening when, and drives the critical path.

An S curve compresses the whole project into one cumulative line. It answers whether the project as a whole is on track for progress and cash flow. Builders usually need both: the Gantt chart to manage the work, and the S curve to manage the money and report to the client.

• Loading cost evenly across activities. Real spending is lumpy. Front-loaded items such as cranes and long lead orders distort an evenly spread curve.

• Not rebaselining after major change. After large variations or an extension of time, a stale baseline makes every month look bad.

• Comparing actual cost to plan only. Without earned value, you cannot tell whether overspend is because work is ahead or because it is costing more.

• Ignoring the flat tail. Margin often erodes in the final stages, because preliminaries keep running while progress slows.

• Using claimed value as progress. Claims can be front-loaded, so check the earned value line against what is physically built on site.