1.Primary Sources of Variation

Process variation, the enemy of true Six Sigma process capability, has been shown to have three main causes:

- Insufficient process capability caused by processes which produce relatively high numbers of - defective units
- Inadequate design margin, which results in unnecessarily and unreasonably tight process specifications (sometimes tighter than the customer requires)
- Unstable parts and materials, usually caused by vendors who are not able to control their own processes and ship materials which in turn yield variation in the customer's processes_