S-curve (project controls)

An S-curve is the cumulative plot of a project's value over time - typically three lines: Planned Value (the baseline), Earned Value (progress) and Actual Cost (spend). The shape is an S because projects ramp up, peak, and taper.

Plot cumulative Planned Value week by week and a characteristic shape appears: a slow start while the site mobilises, a steep middle at peak production, and a long taper through commissioning and close-out. Overlay cumulative Earned Value and Actual Cost on the same axes and the whole EVM story fits on one chart.

Reading it is mechanical. The vertical gap between EV and PV is Schedule Variance; between EV and AC, Cost Variance. EV below PV: behind plan. AC above EV: over budget. The horizontal gap between EV and PV at the same height is the schedule slip in time - the idea Earned Schedule formalises.

The curve's power is the trend. A single month's CPI can be noise; three months of AC pulling away from EV is a trajectory, and the divergence is visible to a non-specialist in seconds - which is why the S-curve opens most monthly reports. WBSync draws the baseline PV, EV and AC curves per project (in cost or hours) in weekly buckets, updating as hours are approved.

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