An S-curve is cumulative progress (or cost) plotted against time. Slow start, steep middle, slow finish — the shape appears in almost every project, and deviations from it are the earliest warning system you have.
The three curves that matter
Plot these together, updated monthly:
- Planned (PV) — the baseline distribution of budget over time.
- Earned (EV) — cumulative value of work actually performed.
- Actual (AC) — cumulative money actually spent.
The vertical gaps between them are your variances: EV below PV = behind schedule; AC above EV = over budget. The gap trend matters more than any single month.
Patterns and what they predict
- Earned curve flattening early — progress is stalling. Front-end engineering or procurement is usually the culprit. This shows up 2–3 months before the milestone slips.
- Actual hugging planned while earned lags — the most dangerous pattern: you're spending to plan but not producing to plan. Burn rate looks healthy in finance reports while the project quietly sinks.
- Steep late earned curve in the forecast — the "hockey stick". A recovery plan that needs the last quarter to run 2× historic productivity is a hope, not a plan. Interrogate it.
- Actual and earned parallel but offset — a one-time hit (a variation, a rework event) rather than systemic inefficiency. EAC = AC + (BAC − EV) may be the right forecast formula here.
Building one properly
- Distribute budget over time using the resource-loaded schedule, not a spreadsheet guess. In P6: publish PV from the baseline; EV from activity percent complete against baseline cost.
- Use consistent progress measurement rules (physical % complete, milestone weights) or the earned curve is fiction.
- Show early/late envelope curves (from early dates vs late dates) — a progress line inside the envelope is fine even if below the early-dates plan.
Reporting tips
- One chart, three curves, monthly grid, and a two-line narrative: "what moved, why, what we're doing."
- Add the forecast (EAC distributed over remaining time) as a dashed extension — sponsors think in end dates and final cost, not indices.
- Keep the y-axis honest. Truncated axes hide the story the curve exists to tell.
Generate the underlying numbers with our EVM Calculator, and track them in the budget tracker template.