Tài liệu Six SigmaLean Six Sigma brings together two business improvement methodologies pot

Chapter 2: Six Sigma

Lean Six Sigma brings together two business improvement methodologies: Lean and Six

Sigma. It unites the emphasis on cutting waste from Lean, with a focus on reducing

variation from Six Sigma. It is not entirely clear when or where this combination arose,

although some identify the source as Michael L Georges’ 2002 book entitled “Lean Six

Sigma: Combining Six Sigma with Lean Speed”. Since its publication, the fusion of these

concepts has evolved to become one of the world’s foremost business improvement

techniques.

What is Six Sigma?

Sigma, from the Greek letter (σ), is used by statisticians to measure the variability in any

process; so the performance of an organisation can be assessed by the sigma level of its

business processes. Traditionally, companies have accepted three or four sigma

performance levels as the norm, even though these processes create between 6,200 and

67,000 defects for every million opportunities. Adhering to the Six Sigma standard

generates only 3.4 defects per million opportunities.

Companies operating at three or four sigma performance typically spend between 25 and

40% of their revenues fixing problems. (Cost of poor quality later is covered later.) Those

functioning at the Six Sigma level typically spend less than 5%.

The term Six Sigma process comes from the notion that if there are six standard deviations

between the process mean and the nearest specification limit, as shown in the graph

below, practically no items will fail to meet specifications. Should there be a Four Sigma

process, many items will fall outside the specification.

Six Sigma is used to ensure a product or service is as good as it can. Measuring the Six

Sigma level of a process is a key part of this process. It can be linked to customer

assessments to develop a baseline from which improvements can be implemented and

measured. If the level of variation in the product or service can be reduced, this will cut

costs for both the company and the customer, as performance moves towards the

optimisation point.