Common and Special Cause Variation

Yesterday I mentioned about output vs outcome, and how I prefer to focus on output instead of outcome. The reason being that there are many factors that influence outcome.

Some of those factors are things that you can control. Many are things that you cannot.

Case in point: I recently completed a training last month that got one of my highest ratings recently. Time to celebrate right? But here is the thing, I did the same training a few months ago, and it got a poor rating that time. I did not change anything between the classes - same trainer, same training, same content.

The reason could be anything. Maybe one set of participants was more enthusiastic and engaged. Maybe someone in the training was expecting something different. And different people learn in different ways, maybe it just didn't click with some participants who turned up in one batch. You do the exact same training over and over and you will get different ratings each time.

This variation in the outcomes due to random factors that are inherent in the system has a name. It's called common cause variation.

On the other hand, maybe something goes wrong one class. There are technical issues or you forget one topic, and this affects the rating. These types of changes in outcome are called special cause variation. With special cause variation you can pinpoint the reason that caused the change in outcome.

Why does it matter to understand the two types of variation? Stay tuned for tomorrow's post.