What is the Difference Between Control Limits and Specification Limits?

Control limits and specification limits are two different concepts in process management and quality control. Here are the key differences between them:

• Control Limits:
• Calculated from process data for a particular control chart, such as an X-bar chart or an Individual measurements chart.
• Represent the voice of the process, indicating the expected variation in the process based on past performance.
• Appear on control charts and are used to guide process actions.
• Help in recognizing when a significant change from the past has occurred, justifying an investigation into the process.
• Specification Limits:
• Defined by the customer, representing the voice of the customer.
• Apply to individual items being measured and appear on histograms, box plots, or probability plots.
• Generally consist of two types: Lower Specification Limit (LSL) and Upper Specification Limit (USL), which quantify the acceptable range for the process outcome.
• Separate good items from bad, indicating whether the product meets customer requirements or not.

In summary, control limits are calculated from process data and represent the expected variation in the process, while specification limits are defined by the customer and indicate the acceptable range for the process outcome. These two concepts are different and serve different purposes in process management and quality control.

Comparative Table: Control Limits vs Specification Limits

The following table highlights the main differences between control limits and specification limits:

Control limits are used in statistical process control (SPC) to establish boundaries for process variation using process data. They help in distinguishing between variations caused by the inherent nature of the process and those that signal a change in the process. On the other hand, specification limits are predetermined goals set before product or process development begins, defining the range of acceptable outputs that meet quality standards. While control limits aim at improving process performance, specification limits focus on customer satisfaction and compliance.