"If the output of a process is centered around a target value, but has a wide spread of values, in other words a large standard deviation, the process is accurate but not precise".
"On the other hand, if the process has a small standard deviation (less variation) but the average value of the process parameter is far from the target, then we could say that the process is precise but not accurate"._
Process Capability Ratios Cp & Cpk
"The Cp helps us to answer the question: Are we able to produce good products?"
"The Cpk ratio helps us answer the question: Are we making good products?".
The Goal of Six Sigma
"How do we translate Sigma in terms of defects?"
The sigma scale of measure is precisely correlated to such figures as defects-per-unit, parts-per-million defectives and the probability of failure/error.
To reduce the consequences of high defect levels, world-class companies now seek process ratings of 6s i.e. 3.4 defects per million opportunities._