Variation comes from two sources, common and special causes. Lets you at this scenario, how long it takes you to get to school in the morning. Maybe it takes you 40 minutes on average. Some days it may take a little longer, some days a little shorter. But as long as you are within a certain range, you are not concerned. The range may be from 35 to 45 minutes. This variation represents common cause variation --- it is the variation that is always present in the process. And this type of variation is consistent, random and predictable. You don't know how long it will take to get to schooltomorrow, but you know that it will be between 35and 45 minutes as long as the process remains the same.
Now, in thecase of special cause variation suppose there was an accident on the road and there was heavy non moving traffic . How long will it take you to get to work? Definitely longer than the 35 to 45 minutes in your "normal" variation. Maybe it takes you an hour longer. This is a special cause of variation. Something is different. Something happened that was not supposed to happen. It is not part of the normal process. Special causes are not predictable, non random and are sporadic in nature.
