To increase process consistency/performance, i.e. reduce its variation, we must start identifying the source of variation and work on ways to mitigate/eliminate it. The sources of functional variation are categorized as follows:

  1. Outer Noise: External factors from the environment.
    Examples: Temperature, change in voltage, human performance.
  2. Inner Noise: Changes in product characteristic.
    Examples: Drift from nominal design due to deterioration, aging.
  3. Variational Noise: Variation from one unit to other.
    Example: Manufacturing processes imperfections.

According to Dr. Deming, the problems that create variation in a process fall into two categories: common causes and special causes.

Common causes: Variations caused by many factors, each of which individually contributes very little to the total variation. Can only be dealt with improvements in design. Management should be responsible.
Examples: Poor/lack of training, inappropriate methods, poor supervision.

Special causes: Variations that are predictable and can be corrected / controlled at the operator/supervisor level.
Examples: broken tools, machine malfunction, poor batch of raw material.

It is extremely important that we understand clearly where variation come from so we know which corrective actions need to be taken, as well as whose responsibility it is to take those actions.