Common Cause vs. Special Cause: The Basics of Process Variation

When dealing with process variation, you’ll need to learn to recognize the two general types of it, and the ways they impact the operation of your business. In most cases, some level of variance to a process is inevitable: the question is whether the factors that cause this variation are controlled and predicted, and if you can account for that variation in your output projections. If you’re able to work around it, there’s usually no big issue but things tend to get messier once special cause variation is introduced to the mix.

Common Cause Variation

Common cause variation is variation resulting from factors that may or may not be known, but the final impact they have on your output is predictable and controllable, and you would usually know to expect some variation in specific areas when looking at your final reports. As we said above, some level of variation is inevitable for most types of processes, and this is usually the kind you want to have if you want to be sure that you’re in control of the current situation.

Note that in some cases, common cause variation can still be quite high, and this might cause issues in your production. Take steps to reduce it if you are able to identify the root causes, but don’t stress too much if you can’t get to the bottom of the situation.

Special Cause Variation

On the other hand, special cause variation occurs when something out of the ordinary happens in a process. This might be a one-time occurrence, or it can develop into a repeating effect, in which case it might eventually be categorized as common cause variation depending on the circumstances. It’s important to understand that sometimes you will simply not be able to avoid special cause variation, either because the nature of a process is prone to it, or because of external factors in your environment that are beyond your control.

When you’ve identified special cause variation though, it’s important to take prompt steps to deal with it. It may not be such a problematic factor right now, but if you leave it unattended for a long time, this might be a real problem that can bring down some parts of your operations. There are some tools that can help you figure out where some issues are coming from, but you may need to come up with some solutions of your own in some cases, depending on the complexity of the situation.

Identifying Both with the Appropriate Tools

This brings us back to the most important point you need to be aware of what tools are available in your area of work for dealing with problems like that, and know how to apply them to your current situation in order to figure out where different types of variation are coming from, and develop an adequate solution. Special cause variation is typically easier to uncover as it will deviate from the norm more strongly, but there might be some exceptions. On the other hand, when dealing with common cause variation, you might have access to more historic data that can give you a better idea of what’s going on with the help of some statistical analysis.

Once you’ve learned what common cause and special cause variation both are, and how they relate to your line of work, you should be more prepared to identify and deal with them, and you will find them less problematic in the long run as impact on your operations will be minimized and kept to a controllable state where you can drive things in the right direction.