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Warning limits on control charts are limits that are inside the control limits. You can define Upper Warning limits and lower warning limits. When warning limits are used, control limits are referred as action limits. By default, warning limits are set at 2-sigma. Advantages of using warning limits are they signal process changes more quickly than the 3-sigma action limits. WECO rule is considered for a process when 2 of 3 data points are between 2-sigma and 3-sigma of the chart centerline.

In control charts:

  • If the mean lies within warning limits, no action is taken.

  • If the mean lies between warning and action limits, take another sample

  • If the mean lies outside action limits, take action