S-Curve

What it plots

Time runs along the horizontal axis, cumulative value up the vertical. The plan line is accumulated PV; the actual line is accumulated EV; organizations running cost EVM overlay an AC curve on the same chart. The vertical gap between plan and actual is schedule variance (SV); the horizontal gap at equal height offers a time-based reading of the delay (the Earned Schedule family of measures) — an estimate leaning on the PV curve's slope and activity mapping, not a settled count of days late. Being cumulative matters: absent corrections to the baseline or the actuals, the running total does not, in principle, decrease — so the curve's slope is that week's speed, and a flattening slope is a warning that fires before the gap even widens. Tools that allow downward progress corrections or value-axis changes can, however, show dips in the actual line.

Why it matters

It turns trend into a single page. A table speaks only of this week's state; the curves show whether the gap is opening or closing. In executive reporting, one S-curve reads faster than dozens of progress rows. The shape itself is information: a plan line steep at the start has stacked its risk up front, and an actual line that jumps in stair steps means completion judgments arrive in large lumps.

Common misconceptions

Reading "actual touched plan" as "problem over" is the misconception: the value gap may close while the cost that closed it stays off this chart (the cost axis is CPI's question). And an S-curve says how far apart the lines are, never why — causes are found by descending into the WBS to see which tasks slipped. The curve starts the conversation; it does not end it. Plot discipline matters too: sparse snapshots leave a few dots instead of a curve, and the whole advantage — showing trend — evaporates.