What it means
A construction manager wants to see whether work is progressing as planned, and a monthly total alone hides the trend. A cumulative curve shows how costs or completed work build over the project life, and each point includes all prior periods, so the curve generally does not fall unless data is corrected or reversed.
APM defines a cost curve as cumulative cost against time and separates planned, actual and earned value concepts, and an S-curve can display one or more of those series. Choose the measure and label the vertical axis, because planned spending, actual spending and percentage of work complete answer different questions.
Set a time-phased baseline showing what should have been achieved or spent by each date, and record actual costs or hours consistently, since late supplier invoices can make the curve appear artificially low. Earned value expresses the budgeted value of work actually completed, which is not the same as cash paid.
Many projects show a slow start, a steep middle and a slow finish. Design, permits and setup can precede heavy delivery, major construction or deployment can make spending and progress accelerate until capacity constraints flatten it, and testing and handover may slow measured progress near completion even as small costs continue.
Not every project follows that pattern, and some have expensive equipment upfront or long testing at the end, so the cumulative pattern may not look like an S. Compare like series.
Planned cost versus actual cost tells whether spending differs, not whether work is ahead or behind, so use earned value for context. Cost variance is earned value minus actual cost, where a negative result suggests overrun, and schedule variance is earned value minus planned value, which indicates whether budgeted work accomplished is above or below plan and is not calendar days late.
Investigate sudden jumps, because a supplier payment, milestone recognition or data upload can produce a sharp step that may not represent physical progress. Watch scope changes, as an old baseline may no longer be a fair comparison once the project grows, and record approved changes.
A gap between curves can prompt a reforecast, so show the baseline and latest forecast separately. Use the right granularity and units, since weekly points may help a short project while monthly points may fit a multi-year programme, and a percent-complete curve should not be plotted against currency without separate axes.
The chart shows divergence, not the root cause, as a slow curve may arise from one blocked critical task, and subjective progress estimates can look precise without being reliable. For owners, the S-curve is a trend and control tool for asking why the project diverges, then inspecting schedule, scope and work quality.
In practice
Real-world examples.
Example
A building project plots planned and actual cumulative spending by month. The two curves track each other until month 5, when the actual line pulls above plan after an early steel purchase.
Example
A software rollout compares budgeted work completed with the original plan. The earned value line sits below the plan, showing that testing is behind schedule even though spending is on budget.
Example
An equipment purchase creates an early cost step rather than a smooth S shape. The project manager explains the step on the chart so the board does not read it as unusually fast progress.
Formula
Calculation
Cost variance = earned value - actual cost. Schedule variance = earned value - planned value. Negative results indicate overrun and behind-plan work respectively under earned value management.
Worked example: at the end of month 6, work completed has a budgeted value of $4,500,000, actual cost is $5,000,000 and planned value is $5,200,000. Cost variance = $4,500,000 - $5,000,000 = -$500,000, and schedule variance = $4,500,000 - $5,200,000 = -$700,000.
Cost performance index = $4,500,000 / $5,000,000 = 0.90, meaning each dollar spent has produced 90 cents of budgeted work. All of this is separate from the curve shape, and the S-curve simply plots these cumulative values month by month.Case study
Seen in the real world.
Entirely fictional case: Harbor Build sees its actual-cost curve rise above plan. The manager checks earned value and finds that completed work is also behind its planned curve. A supplier invoice timing difference explains part of the spending gap, while a blocked permit explains the progress gap. The chart prompted investigation rather than a single automatic conclusion.
The manager reissues the forecast with the permit delay and shows the baseline and the revised line separately, so the board can see what changed. A scope change for an extra loading bay is also logged as an approved baseline change, which removes a second apparent overrun. Two months later the permit is granted, the earned value line steepens and the gap to plan begins to close. The S-curve becomes a standing item in the monthly report, always with a short note explaining each divergence.
Watch out
Common mistakes.
- Treating low actual spending as proof the project is ahead.
- Plotting different units without labels.
- Assuming every project should show a smooth S shape.
Questions
People also ask.
What is an S-curve?
A plot of a project's cumulative cost, hours or progress over time.
Why use this chart?
It shows how planned and actual totals build and diverge as work progresses.
Does spending alone prove progress?
No. Spending needs comparison with work completed and the plan.
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