Difference Between Control Limits and Specification Limits

Control Limits vs Specification Limits

If a layman looks or hears the words control limits and specification limits, he will probably get nothing out of them, but the same words mean a lot to those involved in production process in a factory. There are many who confuse between these concepts despite them being totally dissimilar. In fact, it is hard to find a relationship between specifications limits and control limits. However, to solve the riddle, this article takes a closer look at the two interesting concepts called control limits and specification limits.

Basically, specification limits pertain to the order of the customer, whereas control limits refer to the variations in the production process that are permissible and crop up during production. Before that we need to know a bit about specifications. These refer to deviations that are permissible from the target, or the end product that we are aiming. Target and nominal are two terms that are often encountered in this connection. While target is obviously the end product that we are aiming, nominal refers to what could be ideal for us. In normal circumstances, nominal and target are same, but we are also aware that there are bound to be variations, which is why specifications limits are set before the start of the process. If we are selling powder milk, we know that we need to fill up to a certain amount in a pack every time but sometimes the amount goes up while in others, the amount slightly goes down. To avoid possible fines because of lesser amount, we set the target as higher than nominal. The specification limits are set in such a manner that the losses to the consumers as well as producer are at minimum.

Control limits, on the other hand, are based upon past performances. You can calculate these limits, and they tell you the variations the process is liable to produce in due course of time and production. Limits of variation arising from a process are referred to as control limits when the process is under statistical control. This is an important point as it tells us that all variations in the process result from a common cause. Whenever there is a huge variation, it is because of a special cause.

Specification limits are normally in a band with two extremes being the upper specification limit and lower specification limit. These USL and LSL are set by the customer and as long as supplied product falls within this range, customer’s expectations are met.