Common cause and special cause

Okay kiddo, imagine you have a toy box with many toys in it. Sometimes you just grab a toy and play with it, and sometimes you choose a specific toy for a special reason, like building a tower with blocks.

Now imagine that the number of toys in the box changes suddenly. Maybe someone gave you more toys, or some toys were taken away. This sudden change is called a special cause.

But let's say you keep playing with the toys and notice that sometimes they are not as fun to play with as they used to be. Nothing has changed with the number of toys, but the way you play with them has changed or maybe some of the toys are starting to wear out or break. This is called a common cause because it affects all the toys and happens over time.

In the grown-up world, common causes and special causes are terms used in quality management to understand why a process is not working as expected. Common causes are the problems that happen all the time and affect the system in the long run, while special causes are problems that happen just once or due to a specific event or change. By identifying and addressing these causes, people can find ways to improve systems and make them work better.