SIX SIGMA FUNDAMENTALS - Software Design for Six-Sigma: A Roadmap for Excellence - page 149

TABLE 7.2 Sigma Scale

Sigma DPMO Efficiency (%)

1 691,462 30.9

2 308,538 69.1

3 66,807 93.3

4 6,210 99.4

5 233 99.98

6 3.4 99.9999966

distance in standard deviation units from the mean to a specific limit. Six Sigma
is a representation of 6 standard deviations from the distribution mean. But what
does this mean? What is the diffence between 6 sigma and 4 sigma or 3 sigma? Six
Sigma is almost defect free: “If a process is described as within six sigma, the term
quantitatively means that the process produces fewer than 3.4 defects per million
opportunities (DPMO). That represents an error rate of 0.0003%; conversely, that is
a defect-free rate of 99.9999966% (Wikipedia Contributors, 2009; Section: Holistic
Overview, para 5).” However, Four Sigma is 99.4% good or 6,210 DPMO (Siviy
et al., 2007). This does not sound like a big difference; however, those are defects that
will be encountered and noticed by the customers and will reduce their satisfaction.
So to point out briefly why a Six Sigma quality level is important is simple; this
company will definitely be saving money, unlike most companies who operate at a
lower sigma level and bear a considerable amount of losses resulting from the cost
of poor quality, known as COPQ. Table 7.2 shows how exponential the sigma scale
is between levels 1 and 6.

7.3 WHAT IS SIX SIGMA?

We all use services and interact with processes each day. When was the last time
you remember feeling really good about a transaction or a service you experienced?
What about the last poor service you received? It usually is easier for us to remember
the painful and dissatisfying experiences than it is to remember the good ones. One
of the authors recalls sending a first-class registered letter, and after eight business
days, he still could not see that the letter was received so he called the postal service
provider's toll-free number and had a very professional and caring experience. It
is a shame they could not perform the same level of service at delivering a simple
letter. It turns out that the letter was delivered, but their system failed to track it.
So how do we measure quality for a process? For a software performance? For an
IT application?
In a traditional manufacturing environment, conformance to specification and
delivery are the common quality items that are measured and tracked. Often, lots are
rejected because they do not have the correct documentation supporting them. Quality
in manufacturing then is conforming product, delivered on time, and having all of