Statistical Process Control 101

Specification and Control Limits

Specification limits are boundaries set by a customer, engineering, or management to designate where the product must perform. Specification limits are also referred to as the “voice of the customer” because they represent the results that the customer requires. If a product is out of specification, it is nonconforming and unacceptable to the customer.

Remember: The customer might be the next department or process within your production system.

Control limits are calculated from the process itself. Because control limits show how the process is performing, they are also referred to as the “voice of the process.” Control limits show how the process is expected to perform; they show the variation within the system or the range of the product that the process creates.

Control limits have no relationship to specification limits.

If a product is outside the control limits, it simply means that the process has changed; the product might be in or out of specification. The shift could be caused by a decrease or increase in variation but has no relation to the specification limits.

Control limits are typically set to +3 standard deviations from the mean. For variable data, two control charts are used to evaluate the characteristic: one chart to show the stability of the process mean and another to describe the stability of the variation of individual data values.

Control limits must never be calculated based on specification limits.

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