Paste your data, get a control chart
Paste a column of numbers, or rows of subgroups, and get a real control chart: limits from the data, Nelson rules 1–4 evaluated, out-of-control points marked.
Nothing you paste leaves this calculation. It is not stored, not logged and not sent anywhere else — the numbers are computed and returned, and that is all.
What the chart is telling you
The centre line is the process average and the outer lines are three sigma either side of it, computed from short-term variation. A point outside them is unlikely enough to be worth investigating. So are several patterns that stay inside them, which is what the run rules catch.
Rules evaluated here
- 1 — beyond 3σ. One point past a control limit.
- 2 — shift. Nine in a row on the same side of the centre line.
- 3 — trend. Six in a row rising, or six falling.
- 4 — alternating. Fourteen in a row zig-zagging up and down.
Rules 5 to 8 — the zone rules — are deliberately not applied. They assume the data is roughly normal and that consecutive readings are independent, and a pasted sample cannot be assumed to be either. Firing them here would paint red points on somebody's data that mean nothing. In the product you switch them on knowingly, per characteristic.
The part most people get wrong
These limits are computed from this data, which is exactly what you want for a one-off look and exactly what you do not want on a live chart. On a real process you lock limits to a stable baseline window, so that later drift shows up as drift instead of quietly widening the limits until nothing ever signals again.
Want this to keep itself up to date?
A calculator answers for the data you pasted. A control chart answers for the data your line produced this morning — limits frozen at a baseline you locked, rules evaluated on every new reading, an alert when one trips.
Other free tools
- Cp / Cpk calculator — Paste measurements — or type a mean and a sigma — with your tolerance, and get Cp, Cpk, Pp, Ppk, the sigma level and the expected parts per million out of spec.
- Control limit calculator — Give it subgroups or individual readings and it returns the control limits for the chart, the range or sigma chart beneath it, and every constant it used to get there.
- Control chart constants — The whole table, n = 2 to 25, with the formula each constant belongs to. The same values the charts on this site are computed from.