"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._