Have you tried to explain the concept of process capability to someone who then looks at you like you have two heads? Try using the following analogy to explain the subject more easily.

Driving in a Construction Area

Imagine you are driving on a highway that is undergoing construction. The width of your vehicle – and because you are a successful Six Sigma practitioner, you have many vehicles available to you – is the variability of your process (i.e., the ± 3 sigma from the mean in a normal distribution). The lines defining your driving lane are your specification limits.


On this day you are driving a motorcycle. You are greeted by one narrow lane with modular concrete barriers on both sides. Driving in the middle of your lane you have plenty of room on either side. You can even wander from side to side and still have room without getting too close to the barriers.


On Tuesday, you take your compact car. This time you do not have as much room between the barriers – only a couple of feet on either side of the vehicle. This is not a problem, but you do have to be a bit more careful of going into and beyond the barriers or, in process capability speak, out of specification.


Now the fun begins. On Wednesday you hop into an 18-wheeler for the same trip. This time you have hardly any room on either side of your vehicle. If you do not stay centered in your lane you run the risk of causing damage to you and your vehicle. Would you try to go the speed limit through these barriers or would you slow down? In process speak, if you are filling your tolerance zone with process variation even when your process is centered, might it also be a good idea to slow your process down?


Just when you are feeling better about getting through Wednesday’s drive, you are asked to drive the same 18-wheeler, but this time you will be towing a manufactured home behind you! You are extremely nervous now – not only do you have almost no room on either side of your vehicle but you also must tow something that takes up more room than the entire driving lane. You are outside the lane (out of specification) before you start to drive. It’s clear now why the 18-wheeler will be accompanied by a vehicle with warning signs and lights on it – its job is not only to warn oncoming traffic, but also to help identify potential hazardous situations (like the barriers). Today you have to find an alternate route. Consider the warning vehicle to be a company’s quality department. It is looking out for both you and your customer.


You take the day off to recover from all the stress of this week’s driving.

Process Potential – Cp

What you have experienced is varying degrees of capability, ranging from good capability (the motorcycle with room to roam inside the lane) to poor capability (18-wheeler towing a manufactured home taking up more room than is available).

Assume that the modular concrete barriers around you are 10 feet wide. The vehicles are average widths:

Vehicle Type Specification
Motorcycle 3 feet
Compact car 6 feet
18-wheeler 9.5 feet
Manufactured home 14 feet

All the vehicles are running straight down through the lane – your process is centered on nominal (the target value). This is equivalent to what is typically called Cp – your process potential – calculated as the width of your specification (the vehicle) divided by your variability (the barrier). Using the values previously noted, the vehicle capabilities are:

Vehicle Type Specification Capability
Motorcycle 3 feet 3.33
Compact car 6 feet 1.67
18-wheeler 9.5 feet 1.05
Manufactured home 14 feet 0.71

Which vehicle would you prefer to drive? The one with the highest capability – the motorcycle.

Process Performance – Cpk

Now consider a variation. In this example, you are running your vehicle (or process) off center, thereby getting closer to the barrier (specification limit) on one side. As you approach the barrier, your opportunity for success is lowered. This is your Cpk – process performance – the distance from the center of your vehicle to the closest barrier, divided by the distance from the center of your vehicle to the edge of the vehicle closest to that same barrier. The reason why you only use the closest side is that this is the only one in danger of being damaged.

In this situation a motorcycle running 3 feet away from the barrier would have a capability of 2.0 while the compact car would have a capability of 1.0. You do not want to be that close – you may be scraping the paint off the side of your car. In a production environment, you are at risk of sending defective product to your customer.

With the 18-wheeler you do not have any wiggle room; the manufactured home would be ruined if you attempted to go through the barriers – intentionally or accidentally. If your process has a Cp of less than 1, even if your process is centered, you will make bad product. That leaves only two options:

  1. Move out the barriers – that is, widen the specification limits. Not many customers are willing to widen the specification limits; they are what you agreed to initially.
  2. Reduce your variation so that your process fits between the existing barriers. The manufactured home industry has figured out how to get around this dilemma – they ship the homes in sections, literally cutting their variation in half.

Now you should be able to explain process capability to anyone who is new to the concept. And remember to center your processes, reduce your variation and increase your process capabilities.

Centered and Off-centered Performance
Centered and Off-centered Performance

This article was originally published in Quality Progress.

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