Process control vs. process capability

A process that is in control is one that only shows common-cause variation, without the presence of any special causes. If special causes appear, the process is considered out of control. Remember, SPC (Statistical Process Control) charts are tools that simply show variation in the process and help you determine whether the process is stable or not. Control limits are determined by the behaviour of the process itself—no one can arbitrarily choose where they should be. They represent the “voice of the process,” not what you or I would like them to be.

On the other hand, process capability is about how well the process performs relative to customer requirements, or specifications. These specifications represent the “voice of the customer.” By graphing a histogram of process variation and comparing it to the spec limits, you can assess whether the process is meeting expectations. If the Cp value is 1 or greater, the process is considered potentially capable—meaning the spec limits are wider than the process variation.

It’s important to note: process control and process capability are not the same.

So, what are the possible combinations between these two?

Let’s take a look at a simple diagram with four quadrants that explain this relationship. Starting with the down left quadrant: this is where the process is both in control and capable. That’s the ideal scenario—what everyone strives for. If the process variation is small compared to the specification limits, great! But even if the variation is slightly larger (yet still within limits), it’s important to closely monitor the process to make sure the mean stays centred.

Now to the top left quadrant—here, the process is in control but not capable. How does that happen? Imagine you’re driving steadily at 60 km/h , but you’re going in the wrong direction. You’re in control, but you won’t reach your destination.

Within this quadrant, there are two cases: one with small process variation and one with large. Even with small variation, if the process mean is off target, you’ll still end up producing defects. Think of parking a small car in a large garage, but always steering too far left and hitting the wall—you’re consistent, but not successful. In this case, the process needs to be realigned so that the mean is centred between the spec limits.

If the process variation is too wide compared to the spec width, it’s like trying to park a massive truck in a small garage—it just doesn’t fit. What needs to happen? You’ll need to reduce variation through quality improvement efforts, like a Six Sigma DMAIC project. And of course, make sure the process mean is on target.

Let’s move to the top right quadrant: this is when the process is both out of control and not capable. What should you do? First, remove any special causes to bring the process back under control. Then, focus on reducing variation and improving the process using tools like Six Sigma DMAIC. This is the worst-case scenario, but unfortunately, it’s a common one. The good news? It’s full of improvement opportunities.

Finally, the down right quadrant—where the process is out of control. Remember: an out-of-control process is unpredictable, and by definition, not capable. That means this quadrant technically doesn’t exist. It’s only included as a teaching tool to reinforce the connection between control and capability. Once again—if there’s no control, there can be no capability.

These are the possible combinations of process control and process capability. Your goal? Bring the process under control, reduce variation, and align the process mean with the target. That’s how you move from a stable process to a truly capable one.