Control Limits vs. Specification Limits

Attribute	Control Limits	Specification Limits
Definition	Statistical limits used to determine if a process is in control or out of control.	Predefined limits set by the customer or product requirements.
Purpose	To monitor and maintain the stability of a process.	To ensure that the product or process meets specific requirements or standards.
Usage	Applied during the process control phase.	Applied during the product or process design phase.
Calculation	Based on statistical analysis of process data.	Based on customer specifications or regulatory requirements.
Nature	Variable and adaptable based on process performance.	Fixed and predetermined based on product requirements.
Focus	On process stability and variation.	On meeting specific product or process requirements.
Control	Within control limits indicates a stable process.	Within specification limits indicates a conforming product or process.
Out of Control	Process is considered out of control if data points fall outside control limits.	Product or process is considered non-conforming if measurements fall outside specification limits.

Introduction

Control limits and specification limits are two important concepts in quality control and process improvement. While they both involve setting boundaries for a process, they serve different purposes and have distinct attributes. In this article, we will explore the characteristics of control limits and specification limits, highlighting their similarities and differences.

Control Limits

Control limits are statistical boundaries used to monitor and control a process. They are typically derived from process data and are based on the inherent variability of the process. Control limits are used in statistical process control (SPC) to determine whether a process is in a state of statistical control or if it is exhibiting special cause variation.

Control limits are calculated using statistical methods such as the mean and standard deviation of a process. The most commonly used control limits are the upper control limit (UCL) and the lower control limit (LCL). These limits are typically set at three standard deviations from the process mean, resulting in a control chart with a 99.73% confidence level.

Control limits help identify when a process is experiencing common cause variation, which is the inherent variability of the process. When data points fall within the control limits, it indicates that the process is stable and predictable. Any data points outside the control limits suggest the presence of special cause variation, which could be due to assignable causes that need to be investigated and addressed.

Control limits are dynamic and can be recalculated periodically as new data becomes available. This allows for ongoing monitoring and adjustment of the process to maintain its stability and performance. Control limits are primarily used by process owners and operators to ensure that a process is operating within acceptable limits and to detect any deviations that may impact product quality or performance.

Specification Limits

Specification limits, also known as tolerance limits, are predetermined boundaries that define the acceptable range of a product or process characteristic. Unlike control limits, which are based on process data, specification limits are typically set by customer requirements, regulatory standards, or engineering specifications.

Specification limits are often represented by an upper specification limit (USL) and a lower specification limit (LSL). These limits define the range within which a product or process characteristic must fall to meet the desired quality or performance criteria. Any values outside the specification limits are considered non-conforming and may result in product rejection or customer dissatisfaction.

Specification limits are fixed and do not change unless there is a deliberate decision to revise them. They are determined based on factors such as customer expectations, safety requirements, legal regulations, or engineering constraints. Specification limits are typically wider than control limits to allow for natural process variation while still meeting the desired quality or performance standards.

Specification limits are primarily used by quality control personnel, product designers, and engineers to ensure that a product or process meets the specified requirements. They provide a clear indication of whether a characteristic is within the acceptable range or if it requires adjustment or corrective action.

Comparison of Attributes

While control limits and specification limits serve different purposes, they share some common attributes. Both limits are used to define boundaries for a process or characteristic, and they help in monitoring and improving quality. However, there are several key differences between the two:

1. Purpose

Control limits are primarily used to monitor and control a process, ensuring its stability and predictability. They help identify when a process is experiencing special cause variation that requires investigation and corrective action. On the other hand, specification limits are used to define the acceptable range of a product or process characteristic, ensuring it meets the desired quality or performance criteria.

2. Calculation

Control limits are calculated based on process data, typically using statistical methods such as the mean and standard deviation. They are dynamic and can be recalculated periodically as new data becomes available. In contrast, specification limits are predetermined and set based on customer requirements, regulatory standards, or engineering specifications. They are fixed and do not change unless intentionally revised.

3. Variability

Control limits are based on the inherent variability of a process and are typically set at three standard deviations from the process mean. They account for common cause variation and allow for natural process fluctuations. Specification limits, on the other hand, are wider than control limits to accommodate both common cause and special cause variation. They provide a range within which a characteristic must fall to meet the desired quality or performance standards.

4. Focus

Control limits are primarily focused on the process itself and its stability. They are used by process owners and operators to ensure that a process is operating within acceptable limits and to detect any deviations that may impact product quality or performance. Specification limits, on the other hand, are focused on the end product or characteristic. They are used by quality control personnel, product designers, and engineers to ensure that the product or process meets the specified requirements.

5. Control vs. Compliance

Control limits are used to monitor and control a process, aiming to keep it within acceptable limits and prevent excessive variability. They are proactive in nature and help identify potential issues before they impact product quality or performance. Specification limits, on the other hand, are used to assess compliance with predefined requirements. They are reactive in nature and indicate whether a product or process meets the desired standards or needs adjustment.

Conclusion

Control limits and specification limits are both important tools in quality control and process improvement. While control limits focus on monitoring and controlling a process to ensure stability and predictability, specification limits define the acceptable range of a product or process characteristic to meet desired quality or performance criteria. Understanding the attributes and differences between these limits is crucial for effectively managing and improving processes while meeting customer requirements and regulatory standards.