Paste your data, get every rule violation explained
Every run rule evaluated on your data, each violation named in plain English with what it usually indicates — a shift, a trend, tool wear, two machines mixed.
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 rules are for
A point outside the control limits is the obvious signal, and the least useful one: by the time it fires, the part is already made. The run rules exist to catch the process moving while everything is still inside the limits — a shift, a drift, a mixture of two machines — which is where the money is.
Nelson or Western Electric?
Western Electric's handbook (1956) defined four zone tests. Nelson's set (1984) keeps those and adds four more, renumbered — which is why the same test has two names:
| Western Electric | Nelson | The test |
|---|---|---|
| 1 | 1 | One point beyond 3σ |
| 2 | 5 | 2 of 3 beyond 2σ, same side |
| 3 | 6 | 4 of 5 beyond 1σ, same side |
| 4 | 2 | 8 in a row on one side of the centre line |
Pick Western Electric above and the findings are numbered their way, with the Nelson number in brackets. The one place the standards genuinely differ is that last test — Western Electric uses eight consecutive points, Nelson uses nine — and this checker evaluates nine in both modes, so its Western Electric view is very slightly less sensitive than the handbook. It says so on the finding rather than quietly relabelling one standard as the other.
The part most people get wrong
Running all eight rules is usually a mistake. Each rule has its own false alarm rate, and together they fire often enough on healthy data to train people to ignore the chart. The zone rules (5–8) also assume the data is roughly normal and that consecutive readings are independent — on a fast-sampled sensor they fire constantly and mean nothing. Pick the rules that match your process, and be able to say why each one is on.
And the limits must be frozen. If they were computed from the same window you are checking, a drifting process drags them along and the rules never fire at all. This tool computes limits from the data you paste, which is right for a one-off look and wrong for a live process — see the control limit calculator for why.
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 generator — 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.
- Cpk confidence interval & sample size — A Cpk of 1.33 from 30 pieces has a 95% interval of roughly 0.97 to 1.69. See the uncertainty in your own number, and how many parts would settle it.
- Cpk ↔ PPM ↔ sigma level converter — What PPM is a Cpk of 1.33? What Cpk does 3.4 PPM need? Both conventions shown side by side, because the 1.5 sigma shift is why two sources disagree by a factor of ten.
- 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.