Author: Frederick (Ken) Sexe

As engineers we deal with specifications every day. As a result most of our efforts are focused on meeting specifications as a means to meet customer requirements. Yet there is one other set of limits that are just as important as specification limits but for different reasons.

Taiichi Ohno defined these succinctly: he defined process limits as “the voice of the customer” and control limits as “the voice of the process”. Specifications are derived through actions taken to identify and quantify customer needs and apply them to the design. Control limits can only be defined through statistical process control methods that identify how the process accomplishes its intended purpose. This distinction is important as they both provide different information and are defined in different ways.

It sometimes occurs that specification limits are treated the same as control limits and vice versa. This assumption is dangerous as it assumes that the customer requirements are identical to process performance. It can also give a false comfort by providing a false impression that as long as the process operates within customer requirements that it is under statistical control.  A process can operate within control limits yet not fulfill specification limits and vice versa; this distinction is crucial because the actions taken to improve the system are different for each scenario. Taking the wrong action based on a flawed assumption can cause more harm to the performance of the process than before while masking other problems influencing the process overall.
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Frederick (Ken) Sexe is a lifelong learner currently wrapping up his PhD in Engineering Management and Organizational Psychology at Northcentral University. His hobbies include challenging prevailing patterns of thinking that discourage new ideas while developing new ways to do things. He is currently employed as a Senior Systems Engineer at Raytheon where he is taking a career break from management to pursue his educational goals and focus on his family.