Question

1- What is the distinction between common variation and specific variation? Why do we make this distinction.

Answer 1

Common cause variation is predictable, although sometimes it is difficult to predict because it's not always obvious. This is the variation that does the most harm to your process over time, even if it's just little by little. Special cause variation is not predictable. Variation caused by special causes might have a huge impact on your process, but it's a one-off event.

Special cause variation is those one-off instances that can't (and shouldn't) be remedied in your process improvement efforts. For example, let's assume someone stole all the wheels from your delivery vehicles. You wouldn't purchase and store a spare set of tires for every car in your delivery fleet on the off-chance that they might be stolen again sometime in the future. But that's (metaphorically) what happens when people try to solve special cause variation.