What is the Key Difference between the Specification and Control Limits?

When the winds of change blow, some people build walls and others build windmills: Chinese Proverb

Introduction

What is the difference between Success and Joy? Sometimes we use them interchangeably without realizing that there can be some difference. Success is external which is for the outer world. While Joy is internal and is for your inner bliss.   

Objective

Have you heard of the terms Control limits and specification limits? Do these terms confuse you? Are you trying to find out the relation between the two terms? Are you wondering whether these terms are the same or different? Are you confused about when and where to use these terminologies? This blog will provide you with clarity on what these two terms are and the key difference between the two.

Definitions:

Specification Limits are the targets set for the process/product by customer or market performance or internal target. In short, it is the intended result on the metric that is measured. They have discrete values and can be counted for recording and analysis.

Control Limits: are the indicators of the variation in the performance of the process. It is the actual values that the process is operating on. It is the real-time value.

Detailed Information

We often hear control limits and specification limits being discussed as if they are interchangeable. However, control limits and specification limits are completely different values and concepts. What is the relationship between control limits and specification limits? Usually, there is no relationship whatsoever.

The most common mistake is to use specification limit values instead of control limit values on an X-bar chart or an individual chart. Using specifications limits on an X-bar is the silliest error!

A process can be incapable of meeting a specification while remaining in statistical control – we are predictably making our product out of specification. 

There are certain conditions where the customer allows the organization to run the process wherein the process meets the tolerance requirements but not in statistical control. They are

-         The customer is insensitive to variations within the specification

-         The cost involved in improving the special causes is more than the

benefits to the customer

-         The special causes are predictable and consistent

Following are some of the key differences between Specification and Control Limits:

Present Challenges:

1.       How often the organization is clear about the difference between specification and control limits?

2.      Why do many organizations plot specification limits in the control chart to monitor the stability of the process?

References:

IATF 16949: 2016

SPC Manual (AIAG) 2nd Edition

Industry Experts