Variation

Manufacturers seek to produce products that are all high quality. By this we can think of defect free and dimensionally identical.

However, two items produced might look identical, but ultimately they are not. There will be minor variations between them, due to minute differences in things like the materials used to build them such as; the manufacturing process; the time of day or day of year, the people working in the process, variations in the equipment, and so on.

It might take a period of time for all effects of variations to show, but over a long period of time, the total effect of all of factors is experienced and starts to affect the overall variation of the product produced. These sources of variation are called 'common causes', because they are always present.

If all factors remain the same, then the amount of variation remains the same.