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Here are three good reasons for calculating Process Sigma:

 It makes you consider customers’ performance needs. How long are they prepared to wait for their order to be processed?

 It makes it easier to compare the performance of different processes. If different metrics are used for each process, comparison is tricky, and it’s difficult to prioritize improvements. Process Sigma is a standard quality metric that can be used to measure any process.© Martin Brenig-Jones and Jo DowdallFIGURE 1-5: Highlighting defects.

 It supports decision-making on what performance needs to be, rather than using an arbitrary target. For example, 99.9 percent success sounds impressive, but it would mean 100 plane crashes worldwide every day, and one hour without electricity every month.

The Process Sigma value represents the population of cases that meet the CTQs right first time. Sigma values are expressed as defects per million opportunities (DPMO) to emphasize the need for world-class performance.

Not all organizations using Six Sigma calculate Process Sigma values. Some organizations just use the number of defects or the percentage of orders meeting CTQs to show their performance. Either way, if benchmarking is to be meaningful, the calculations must be made in a consistent manner.

Figure 1-6 includes yield figures — the right first time (or ‘first pass yield’) percentage. You can see that Six Sigma performance equates to only 3.4 DPMO.

Calculating the DPMO for your process measures quality using defects rather than defectives. This is an important distinction that requires more explanation.

Let's say your customer has several CTQs relating to an order — for example not just the speed (cycle time) but also accuracy and completeness. Each CTQ represents a defect opportunity. Thus, more than one defect may occur in the transaction. You could have a situation whereby the speed CTQ was met, but the accuracy and completeness CTQs were missed. The outcome would be one defective delivery (as a result of these two defects).

© Martin Brenig-Jones and Jo Dowdall

FIGURE 1-6: Abridged Process Sigma conversion table.

In calculating sigma values for your processes, you need to understand the following key terms:

 Unit: The item produced or processed.

 Defect: Any event that does not meet the CTQ.

 Defect opportunity: Any event that provides a chance of not meeting a customer CTQ. The number of defect opportunities will equal the number of CTQs.

 Defective: A unit with one or more defects.

You can work out your Process Sigma performance against the CTQs as shown in Figure 1-7. We have a sample of 500 processed units. The customer has three CTQs, so we have three defect opportunities. The CTQs are related to speed, accuracy and completeness. We find 57 defects. With software, you can determine a precise Process Sigma value, but with the abridged table in Figure 1-6, find the sigma value that’s closest to your DPMO number of 38000. As you can see, this is 3.3.

© Martin Brenig-Jones and Jo Dowdall

FIGURE 1-7: Calculating Process Sigma values.

There is a small quirk to be aware of here. A Process Sigma and standard deviation are not quite the same thing. This results from Motorola adjusting the sigma conversion tables to reflect the variation they observed in their processes over the long term, after studying it for several years. The adjustment is referred to as a “1.5 sigma shift,” reflecting the extent of the shift in performance. Although this adjustment related to Motorola’s processes, everyone adopting Six Sigma has also adopted the adjusted sigma scale.

When we talk about Six Sigma performance before the adjustment, we’re talking about performance data within plus or minus six standard deviations of the average, which embrace 99.999998 per cent of the data. This is the percentage of cases that are right first time in terms of meeting the requirements of the customer. It equates to 0.002 DPMO! Even with the adjustment, we’re still looking at a truly demanding standard, with 99.999666 per cent of cases right first time in meeting the CTQs.