Control limits and specification limits are two distinct concepts in Statistical Process Control (SPC) that serve different purposes in quality management. Control limits are calculated from actual process data and represent the natural variation inherent in a process. They are typically set at thr…Control limits and specification limits are two distinct concepts in Statistical Process Control (SPC) that serve different purposes in quality management. Control limits are calculated from actual process data and represent the natural variation inherent in a process. They are typically set at three standard deviations above and below the process mean, creating Upper Control Limit (UCL) and Lower Control Limit (LCL). These limits tell us what the process is actually doing and help identify when a process is statistically out of control. When data points fall outside control limits, it signals special cause variation requiring investigation. Specification limits, on the other hand, are determined by customer requirements or engineering standards. They define what the process should produce to meet acceptable quality standards. Upper Specification Limit (USL) and Lower Specification Limit (LSL) represent the boundaries within which a product or service must fall to be considered acceptable. These limits are set externally based on customer needs, regulatory requirements, or design specifications. The key distinction lies in their origin and purpose. Control limits are voice of the process - they describe actual performance. Specification limits are voice of the customer - they describe desired performance. A process can be in statistical control (all points within control limits) yet still produce defects if the process variation exceeds specification limits. Conversely, a process might meet specifications but be out of statistical control, indicating instability that could lead to future quality problems. In the Control Phase of DMAIC, both limits are monitored together. The goal is to maintain a stable process (within control limits) that consistently meets customer requirements (within specification limits). Process capability indices like Cp and Cpk measure the relationship between these two types of limits, indicating how well a controlled process can meet specifications. Understanding this distinction is essential for effective process monitoring and continuous improvement.
Control Limits vs Specification Limits: A Complete Guide
Why This Topic Is Important
Understanding the difference between control limits and specification limits is fundamental to Six Sigma and quality management. Confusing these two concepts can lead to incorrect decisions about process performance, unnecessary adjustments to stable processes, or failure to recognize when a process is producing defective products. This distinction appears frequently on Green Belt certification exams and is critical for real-world application.
What Are Control Limits?
Control limits are statistically calculated boundaries that define the expected variation of a process when it is operating in a stable, predictable manner. They are derived from actual process data and represent the voice of the process.
Key characteristics of control limits: • Calculated using process data (typically ±3 standard deviations from the mean) • Appear on control charts (SPC charts) • Indicate whether a process is statistically stable • Upper Control Limit (UCL) and Lower Control Limit (LCL) • Change only when the process itself changes
What Are Specification Limits?
Specification limits are boundaries set by customers, engineers, or regulatory bodies that define acceptable product or service characteristics. They represent the voice of the customer.
Key characteristics of specification limits: • Determined by customer requirements or design engineers • Define what is acceptable versus defective • Upper Specification Limit (USL) and Lower Specification Limit (LSL) • Independent of actual process performance • Do not change based on process data
How They Work Together
The relationship between control limits and specification limits reveals important information about process capability:
Scenario 1: Control limits within specification limits The process is capable of meeting customer requirements. A stable process will produce acceptable products.
Scenario 2: Control limits wider than specification limits The process is not capable. Even when stable, the process will produce some defective output. Process improvement is needed.
Scenario 3: Process in control but producing defects The process is stable (predictable) but not capable. The process mean may need to be shifted, or variation reduced.
Scenario 4: Process out of control but within specifications Products may currently be acceptable, but the unstable process poses risk for future defects.
Key Formulas
Control Limits for X-bar Chart: UCL = X-double bar + (A2 × R-bar) LCL = X-double bar - (A2 × R-bar)
Process Capability Index (Cp): Cp = (USL - LSL) / (6 × sigma)
Exam Tips: Answering Questions on Control Limits vs Specification Limits
1. Remember the Source Control limits come from process data; specification limits come from customer requirements. Questions often test whether you know who or what determines each type.
2. Know the Purpose Control limits tell you if a process is stable. Specification limits tell you if output is acceptable. These are different questions with different answers.
3. Watch for Trap Answers Exam questions may include options suggesting you should adjust control limits to match specification limits. This is incorrect—control limits are calculated from data, not set arbitrarily.
4. Understand Independence A process can be in control while producing defects, or out of control while producing acceptable products. These concepts operate separately.
5. Connect to Capability When questions mention both types of limits together, they often relate to process capability (Cp, Cpk). A capable process has control limits narrower than specification limits.
6. Memorize Key Terms Voice of the Process = Control Limits Voice of the Customer = Specification Limits
7. Read Questions Carefully Look for keywords like 'stable,' 'predictable,' or 'in control' (pointing to control limits) versus 'acceptable,' 'conforming,' or 'meeting requirements' (pointing to specification limits).
Common Exam Question Types
• Identifying which limits appear on a control chart • Determining who sets each type of limit • Interpreting scenarios where limits have different relationships • Calculating control limits from given data • Explaining what action to take based on limit violations