A control chart plots a statistic such as a subgroup mean, a range, an individual reading or a defect fraction in time order against a centerline and upper and lower control limits, usually set at plus and minus 3 standard errors of that statistic. Points outside the limits, or nonrandom patterns inside them, signal a special cause. The chart type follows the data: X-bar and R or I-MR for measurements, p or c charts for counts and attributes.
Thickness control chart on furnace 12 just tripped rule 1, one point above the UCL, so the lot is on hold.
A chart nobody reacts to is wallpaper. Every chart needs an owner and an OCAP, or it should come off the system.
Also heard as
- Shewhart chart
- SPC chart
- process behavior chart
Related terms
Control limits
Boundaries on a control chart computed from the process's own variation, marking how far routine noise normally reaches.
X-bar and R chart
Paired control charts for subgroup averages and subgroup ranges, the workhorse for measured data sampled in small groups.
I-MR chart
Individuals and moving range chart: the control chart for one measurement per period, such as one batch or one lot.
p chart
Control chart for the fraction of nonconforming units in each sample, built on the binomial distribution.
Western Electric rules
Standard set of run rules that flag nonrandom patterns on a control chart, not just single points past the limits.