What does the gap between the three S-curves show?

Each gap is a variance. When the earned-value curve sits below the planned-value curve, the horizontal gap is schedule lag and the vertical gap is schedule variance in dollars: work that should have been done but is not. When the actual-cost curve sits above the earned-value curve, that vertical gap is cost variance: money spent beyond the worth of the work completed. On our example at the data date, earned value of five point four million trails planned value of six million and sits under actual cost of six point three million, so both gaps open downward and the project reads behind and over.

What is the difference between a baseline S-curve and an actual S-curve?

The baseline S-curve is the plan: the cumulative planned value spread across the schedule before work starts, frozen so it can be measured against. The actual S-curves are what really happened, the earned-value and actual-cost curves drawn period by period as the project runs. The whole point of the chart is the comparison. A single actual curve alone tells you how fast you are spending or progressing, but only laid over the frozen baseline does it tell you whether that pace is ahead of or behind where the plan said you would be.

How do you build an S-curve for a construction project?

Start from the schedule of values and the schedule itself. Spread each line item's budget across the months it is planned to be built, then add the monthly figures up cumulatively; that running total is the planned-value, or baseline, S-curve. As the job runs, plot two more running totals each period: earned value, which is percent complete times budget, and actual cost from the accounting system. The three cumulative lines, read against the time axis, give you the full earned-value picture without a single new number beyond what billing and progress already produce.

Construction Glossary · Earned Value

The ConstructionS-Curve

One picture that holds the whole earned-value story. Three cumulative lines over time, and the gaps between them are your cost and schedule variance.

Definition

A construction S-curve is a graph of cumulative cost or progress over time, named for its lazy-S shape: slow at the start, steep through the middle, and flattening near completion. Plotting the planned, earned, and actual curves together turns the gaps between them into cost and schedule variance you can see at a glance.

Reading the three curves

Our $12.0M project runs 16 months. The planned curve reaches 100% at month 16. At the month-8 data date, the earned and actual curves tell you where things really stand.

Planned (PV) Earned (EV) Actual (AC)

At the data date, earned value ($5.4M) sits below planned value ($6.0M), that vertical gap is the schedule variance of −$0.6M. Actual cost ($6.3M) sits above earned value, that gap is the cost variance of −$0.9M.

Why the shape is an S

Construction spends and progresses slowly at first, quickly in the middle, and slowly again at the end. That rhythm is what bends a straight budget line into a lazy S.

Phase 1

Slow start

Mobilization, submittals, and early procurement move money and progress slowly. The curve is nearly flat, which is normal, not a warning sign.

Phase 2

Steep middle

Production hits full stride. Crews are stacked, materials flow, and cumulative cost and progress climb fast. Most of the budget is earned in this stretch.

Phase 3

Flattening finish

Punch list, commissioning, and closeout take time but little money. The curve levels off as the last few percent of work is chased down.

Every gap is a variance

Gap on the chart	What it is	Example reads
Planned above earned (vertical)	Schedule variance, planned work not yet done	−$0.6M
Planned above earned (horizontal)	Time lag, how far behind the pace	roughly two weeks
Actual above earned (vertical)	Cost variance, spend beyond work earned	−$0.9M
Projected end of actual curve	Forecast finish above the baseline	about $14.0M

Extending the actual curve to its projected end is the estimate at completion.

How POD draws the S-curve

POD builds the baseline curve from the frozen budget and plots the earned and actual curves period by period from the same figures its indices use, so the chart and the numbers never disagree. Because the baseline is versioned, an approved change re-shapes the plan line without erasing the original, and the gaps you read on the curve are the same cost and schedule variances POD computes.

The full EVM guide

Frequently asked questions

The whole story in one chart

The S-curve is where earned value becomes a picture. POD draws it from the budget and progress you already report, baseline and forecast included.

Read the EVM Guide Back to the Glossary