Control Chart

The Control Chart is used in Statistical Precess Control (SPC) to monitor a production process.

The control chart can help

• To prevent that a process is going out of control.
• To determine the range of a process.
• Monitor change in process over time (wear out of production equipment).
• Snapshot of a production period.
  

UCL and LCL are not the specification limits!!


Data File

Common Control Chart interpretation (WECO) rules

• A point falls outside the UCL or LCL (3 sigma lines).
• Two out of three successive points are outside the 2 sigma line (second gray line under or above the mean).
• Four out of five successive points are outside the 1 sigma line (first gray line under or above the mean).
• Eight successive points are on one side of the mean line.

When one of these rules happens it will be visible as a red dot in the plot. If needed corrective actions must be taken to restore the process. Keep in mind a point outside the ULC and LCL will can occur in about 1 in the 371 data-points.

Calculations

UCL = Mean + 3 x STDEV
LCL = Mean - 3 x STDEV

Links

• https://en.wikipedia.org/wiki/Control_chart
• https://en.wikipedia.org/wiki/Western_Electric_rules
• http://www.itl.nist.gov/div898/handbook/pmc/section3/pmc31.htm