Process Capability: Understanding the Concept

Help employees grasp the basics of process potential and performance

by Jack Meagher

Have you ever tried to explain process capability to someone only to have that person look at you as if you had two heads? Consider the following analogy when you need to help employees understand process capability.

Understanding the concept

Think about driving different types of vehicles on the highway during a construction period. Imagine the vehicle's width as the variability of the process (the +3 sigma) and the construction barriers on either side of the lane as specification limits.

On Monday, you find yourself driving a motorcycle through one narrow lane with construction barriers on both sides. You have plenty of room on either side of the lane and can wander from side to side without coming too close to the barriers.

Tuesday you take the same route, but you're driving your compact car instead. With only a few feet on either side of the vehicle, there isn't as much room in the lane. This isn't a serious problem, but you have to be a bit more careful of hitting the barriers or going out of specification.

Wednesday you hop into an 18 wheeler for the same trip. This time there's hardly any room on either side of the lane. If you do not stay centered in your lane, you run the risk of causing damage to yourself or someone else. You make it through the trip but are asked to come back the next day and drive the same 18 wheeler towing a manufactured home behind you.

You are very nervous on Thursday. A vehicle with warning signs and lights drives ahead of you to caution oncoming traffic. This warning vehicle is just like your quality department--it's looking out for both you and your customer.

Not only is there no room on either side, but you are towing something wider than the entire driving lane. You realize that you are out of specification before you even get to the narrow lane.

Friday you take off to recover from the stress.

Add math to the correlation

By studying this correlation, employees become familiar with varying degrees of capability. The analogy takes the student from good capability (the motorcycle with room inside the lane) to poor capability (the 18 wheeler towing a home and needing more space than is available).

Typical values can be added for those who tend to be more mathematically inclined. Consider the lane to be 10 feet across; the motorcycle, 3 feet across; the compact car, 6 feet across; the 18 wheeler, 9.5 feet across; and the manufactured home, 14 feet across.

The equivalent of all the vehicles running through the lane, the process centered on nominal, is Cp) the process potential (or the width of the specification divided by variability. Under these conditions, the motorcycle would have a capability of 3.33 (10/3); the compact car, a capability of 1.67 (10/6); the 18 wheeler, a capability of 1.05 (10/9.5); and the manufactured home, a woeful capability of 0.71 (10/14). Which vehicle would your staff members say they'd rather be driving?

Calculating process performance

Now consider running the process, or in this case the vehicle, off center, thereby getting closer to the barrier on one side. Every time the vehicle approaches a barrier, getting closer to one of the specification lines, the opportunity for success is lowered. This is equivalent to Cpk, the process performance. It's the distance from the center of the vehicle to the closest barrier divided by the distance from the center of the vehicle to its edge closest to the same barrier. You're only concerned with the closest side because it's the only one in danger of being damaged.

For example, the motorcycle running 2 feet away from the centerline would have a capability of 2 because (5 - 2)/1.5 = 2. The center of the vehicle is calculated at 5 feet because the width of the lane measures 10 feet. Subtract 2 feet from this center point as that is the distance from the barrier. To calculate the space between the center of the motorcycle and the motorcycle's edge nearest the barrier, divide the vehicle's width in half. In this instance, this gives a denominator of 1.5.

The compact car moving the same 2 feet toward one of the barriers would have a capability of 1 as (5 - 2)/3 = 1.

This comparison should give people who have a difficult time understanding process capability something to relate to. Now it's up to you to go out, center your processes, reduce variation and increase your process capabilities.

JACK MEAGHER is a senior quality engineer with the HiTech Division of New Hampshire Ball Bearings in Peterborough, NH. He earned a bachelor's degree in nuclear engineering from the University of Massachusetts-Lowell. Meagher is the secretary of the ASQ Granite State Section, an ASQ member and a certified quality engineer.

If you would like to comment on this article, please post your remarks on the Quality Progress Discussion Board, or e-mail them to editor@asq.org.

This is one of the best explanations on process capability and process performance I have ever come across. I'm working with other Six Sigma practitioners to invigorate a quality culture, starting with a few presentations to introduce some concepts to an eager and captive audience.
--Jeanne Parks, 02-17-2015

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