Nothings perfect, as any Sigmaphyte would tell you. But as observed in the example above, a sigma of 6 is about as close as you get to it. That’s only 3.4 failures in a million! It’s a pretty hard number to achieve. Most good processes typically run in the Five – Three Sigma range. If your process is running closer to the Two Sigma level then you have some work to do. Here’s the scale range to get an idea.
- Six Sigma = 3.4 DPMO, or 99.9997% defect-free
- Five Sigma = 233 DPMO, or 99.98% defect-free
- Four Sigma = 6,210 DPMO, or 99.4% defect-free
- Three Sigma = 66,807 DPMO, or 93.3% defect-free
- Two Sigma = 308,538 DPMO, or 69.1% defect-free
- One Sigma = 691,462 DPMO, or 30.9% defect-free
Defining opportunities for faults is the trickiest part. Specific problem identification and solving methodologies have been adapted and are commonly used in Six Sigma evaluations. Simply plugging in your pass and fail results may not be a true reflection of your process sigma. However this practice is commonly used and can give you an overall trending of your process.