Analyzing S-Curves

What an S-curve is

The S-curve is the workhorse visual of project controls — the single most common chart in any status report. It takes the period-by-period data the controls structures collect (Module 4A) and renders it as a picture that anyone, technical or not, can read. Where a table of numbers hides the trend, the S-curve shows it.

Why the S shape — and what curves to plot

The shape comes from how projects spend effort: slowly at first (mobilizing, early design), rapidly through the middle (peak construction), then tapering at the end (commissioning, closeout). On the same axes you typically plot two or three curves:

• Planned (baseline) — The cumulative plan — how value should accrue over time, from the schedule and budget.
• Actual / earned — What's actually been accomplished or spent to date — the curve compared against the plan.
• Forecast (optional) — The projected path to completion from today, given current performance.

Read the S-curve

Drag the reporting month to sweep the data date along the timeline. Watch the planned and actual curves diverge, and read the variance at that point:

Reading S-curves well

The S-curve's power is trend: a single point tells you where you are, but the shape over time tells you where you're heading. A gap that's widening each period is a project losing ground; one that's closing is recovering. Watch the divergence, not just the snapshot — and use it to forecast where the curves will end up if nothing changes.

Nine things to remember

1. An S-curve plots cumulative value against time — planned vs actual.
2. It's the most-used chart in project controls — the picture in every status report.
3. "Cumulative" is the key word — the gap is the amount ahead/behind to date.
4. The S shape comes from slow start, steep middle, tapering finish.
5. Plot planned, actual/earned, and optionally forecast on the same axes.
6. Below the planned line = behind; above = ahead.
7. Watch the trend, not just the snapshot — a widening gap is a project losing ground.
8. It's earned value, drawn — planned/earned/actual curves are PV/EV/AC.
9. Don't plot spend alone — on-budget spending isn't on-schedule progress; plot earned value.