Pre-Control Vs SPC

Pre-control is a simple alternative to traditional control charts that is very effective in shop floor environments.  Pre-control is unique for two reasons: (1) it incorporates the specification limits into the control chart, which most control charts do not, and (2) it uses color-coded zones (green, yellow, and red) to make control chart interpretation very easy for operators.  Pre-control limits are established based on the following –

  • the middle 50% of the chart is the Green Zone, which makes up 50% of the specification tolerance for the feature being measured
  • there is a Yellow Zone on either side of the Green Zone that makes up the remaining 50% of the specification tolerance (25% for each yellow zone)
  • the Red zone is everything outside of the specification limits

There are a few guidelines for building and reacting to pre-control charts, but overall they are quite simple to use.

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