In the first part of this series, some technical comments about six sigma were made – however these comments are largely curiosities as six sigma is really a collection of quality management tools.
Here the attempt is made to work through an example for a diagnostic assay.
The assay selected is a home glucose assay and the first step is to set a goal. Although there are a lot of ways to do this, there is an ISO document (15197) which contains goals. According to this standard, the “minimum acceptable accuracy” goal is:
“Ninety-five percent (95%) of the individual glucose results shall fall within ± 0,83 mmol/L (15 mg/dL) of the results of the manufacturer’s measurement procedure at glucose concentrations < 4,2 mmol/L (75 mg/dL) and within ±20 % at glucose concentrations >= 4,2 mmol/L (75 mg/dL).”
The ISO document says that “the minimum acceptable accuracy criteria are based on the medical requirements for glucose monitoring”.
First a word on terminology. In the ISO world, accuracy does not mean freedom from bias, rather it means freedom from all sources of error and is comparable to total error.
Now here is the problem. If one looks at the 75 mg/dL or less range, the goal states that one should not get values outside of ± 15 mg/dL, based on medical requirements. The problem is that this is stated for 95% of all values. So for an assay that just meets requirements this means that just under 5% of the values could exceed the goal which means that the number of defects one will see is 50,000 per million assays! In six sigma terms, this is close to a 3 sigma process (3.14487). This also means that this ISO document says that a 3 sigma process is acceptable.
|Sigma||Defects per Million|
With 3 sigma goals, one should not expect six sigma processes.