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Control Charts Calculator

SPC control charts online. X-bar R, X-bar S, p chart, np chart, c chart, u chart with WE rules.

Open the Control Charts (X-bar/R, X-bar/S, I-MR, p, c, u) calculator

When to use it

Use to monitor a process over time and detect when it goes out of statistical control. The seven Shewhart-type charts cover the standard SPC needs: X-bar/R (continuous, small subgroups, range), X-bar/S (continuous, larger subgroups, SD), I-MR (individuals + moving range), p-chart (proportion defective, varying n), np-chart (count of defectives, fixed n), c-chart (defects per unit, fixed area of opportunity), u-chart (defects per unit, varying area).

When NOT to use it

Do not use a Shewhart chart when the process is autocorrelated; an EWMA or CUSUM chart is more sensitive. Do not pick X-bar / R when subgroups are large (n above 10); use X-bar / S. Do not interpret a single out-of-control point as a special cause until you have ruled out measurement error.

What you get

Per-subgroup statistic plotted with central line and three-sigma upper and lower control limits; Western Electric / Nelson rules flagged on the chart (rule 1: any point outside the limits; rule 2: 9 in a row on one side; rule 3: 6 in a row trending; rule 4: alternating 14 in a row; etc.); summary table of in/out-of-control subgroups.

Worked example

10 subgroups of n=4 measurements, X-bar / R chart. Process mean drifts up over time.

12.4, 12.5, 12.3, 12.6
12.5, 12.4, 12.5, 12.6
12.6, 12.5, 12.7, 12.5
12.4, 12.6, 12.5, 12.4
12.7, 12.6, 12.5, 12.6
12.5, 12.4, 12.6, 12.5
12.8, 12.6, 12.7, 12.6
12.5, 12.6, 12.5, 12.7
12.4, 12.5, 12.4, 12.6
12.6, 12.7, 12.6, 12.5

Expected output: All points within control limits; no Western Electric rule violations on this baseline.

Source: StatVeda built-in example, paired with the analyse page sample for toolId 'controlchart'.

How to interpret the output

Points inside the control limits with no rule violations indicate a process in statistical control (common-cause variation only). Any rule violation flags a special cause that should be investigated and removed. Control limits are derived from the data and are different from specification limits, which come from engineering or customer requirements.

Common pitfalls

Try it in StatVeda

The Control Charts (X-bar/R, X-bar/S, I-MR, p, c, u) engine runs entirely in the browser. No signup, no install, no data sent to a server. Paste your data, hit Run, copy the output.

Open Control Charts (X-bar/R, X-bar/S, I-MR, p, c, u)

References

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