Variation

Manufacturers aim to produce defect-free products with consistent dimensions. In reality, minor differences occur due to differences in materials, processes, equipment, or working conditions. For example, refer to this illustration:

The overall impact of variations takes a considerable amount of time to become apparent. However, over an extended period, the combined effect of these factors is experienced and begins to influence the overall variation in the product produced. These variations, called common causes, exist consistently within the process. If all factors stay constant, variation does not change.