Control limits vs. specification limits

Control limits are calculated from process data for a particular control chart. An X-bar chart and an Individual measurements chart will have different limits.

Specification limits are chosen in numerous ways. They generally apply to the individual items being measured and appear on histograms, box plots, or probability plots.
Specification limits are the values between which products or services should operate. These limits are usually set by customer requirements.

The table below contrasts control limits and specification limits:

Control Limits	Specification Limits
Voice of the process	Voice of the customer
Calculated from Data	Defined by the customer
Appear on control charts	Appear on histograms
Apply to subgroups	Apply to items
Guide for process actions	Separate good items from bad
What the process is doing	What we want the process to do

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