HKSM
Specification Limits
The allowable range around the process mean on a control chart that represents the customer's required performance for a product or service. This requirement-based band can be wider or narrower than the statistical control limits.
Key Points
- Set by the customer, contract, or standard and expressed as LSL (lower specification limit) and USL (upper specification limit).
- Different from control limits, which are calculated from process variation and show statistical control.
- May be tighter or looser than control limits; a process can be in control yet still produce outputs outside specs, or vice versa.
- Used in capability analysis (e.g., Cp, Cpk) to judge whether the process can consistently meet requirements.
Example
A machining process targets a 10.00 mm shaft diameter. The customer specifies 9.95 mm (LSL) to 10.05 mm (USL) as acceptable. The control chart shows the process mean at 10.00 mm with control limits based on natural variation at 9.92 mm and 10.08 mm. Even if the process is stable (within control limits), parts between 10.05 mm and 10.08 mm are out of spec and must be reworked or rejected.
PMP Example Question
Which statement best describes specification limits in quality management?
- The statistically computed boundaries (about +/-3 sigma) that indicate expected process variation.
- The customer-defined tolerance band that determines which outcomes are acceptable (LSL and USL).
- The approved range of the project budget variance allowed by the sponsor.
- The set of thresholds used to categorize risks on a probability-impact matrix.
Correct Answer: B — Customer-defined tolerance band (USL and LSL)
Explanation: Specification limits reflect customer or standard requirements, while control limits reflect process behavior based on statistical variation.
