Even though the six-sigma capability of a process would theoretically yield a 0. 002 defects per million, black belts consider that it will yield 3. 4 defects per million, equivalent to a 4. 5-sigma capability or a Cpk of 1. 5. Why are the black belts more conservative?

A		Errors in measurements of means and ranges.
B		Errors in calculations of the capability coefficient Cpk.
C		Possible exaggerations on process capabilities by vendors.
D		A possible shift in the mean of the process over time.
E		Wear and tear on the parts as they are used.