Untitled

THE SIGMA QUALITY LEVEL

In Quality Management, there has always been an attempt to find a scientific way to measure quality. Measuring quality helps in setting up quantitative targets for quality improvement. It also helps in comparing the organisation with  benchmarks or best-in-class organisations from quality perspective.

Traditionally, quality of a product or component is measured in percentage e.g. percent defective, percent defect, percent error, percent failure etc. For example, the statement “lot quality of hardware is 1%” implies that the lot of hardware is having, on an average, 1% detectives.

Of late, the quality management in modern companies follow statistical quality control (SQC) techniques and use SQC related terminology. Accordingly, quality level is often expressed as sigma quality level and processes are monitored through control charts which work on three sigma limits.  Sigma is a Greek letter denoting an important statistical parameter called standard deviation.

Today, expressing quality in terms of sigma level  is considered the most scientific way of quantifying the quality level of a product/ process/ component or even the quality level of an organisation!.

ONE SIGMA, TWO SIGMA, THREE SIGMA

A “one sigma quality level” corresponds to 31.73% detectives, “two sigma quality level” corresponds to 4.55% detectives and “three sigma quality level” corresponds to 0.27% detectives. A higher sigma level signifies a better quality. The theory behind these odd percentages has something to do with normal distribution, an important subject in statistical quality control. But not to worry, enough charts and graphs are available online and offline for converting sigma quality level into percent defective and vice versa. Note that sigma level could be fractional too e.g. 2.5 sigma and the like. 

Quality experts and statisticians regard three sigma quality level as “ just OK” for normal processes and find acceptable for most purposes. The entire subjects of statistical process control, control charts and process capability hover around three sigma limits. In fact, so popular is the term three sigma among quality professionals that any person or just anything having abnormal traits, is said to be beyond three sigma limits! 

However, we should always keep in mind that three sigma quality level is a level that is neither too high nor too low. If the organisation wishes to achieve excellence, it must strive to reach higher sigma levels. Having achieved a three sigma level quality, one can always embark on a higher level quality, say, four sigma, five sigma, six sigma and beyond. Note that there is nothing like infinite sigma level (or zero defect). The sigma quality approach believes that zero defect in a population of products/ components is an impossibility. Instead, it emphasises that we should go on reducing the defect level continuously so that sigma level keeps going up. Normally, it happens step-by-step. For example, it will be rather impractical to jump straightaway from three sigma quality level to six sigma quality level.

ABOUT SIX SIGMA QUALITY INTIATIVE

To start with, it must be appreciated that Six Sigma is basically a quality improvement initiative (though some people prefer to call it a business initiative while others prefer to call it a philosophy).

Six Sigma initiative is different from other quality improvement initiatives in that it aims very high. It doesn't aim at something like 1% defectives maximum. It doesn't not aim at 0.27% max defectives either (the well known three sigma limits of statistical quality control or statistical process control). Instead, it demands much more. It aims at a defect rate (or error rate) of 3.4 defects per million opportunities. A very tough target indeed! But it's achievable and many organisations have demonstrated it.

Note that Six Sigma initiative doesn't talk about zero defect because it believes in the bitter truth that zero defect is something of a perfection that can never be achieved. Rather, it targets on a near zero defect level, which can be achieved by rigorous project-by-project approach following the standard DMAIC (or sometimes DMADV) methodology.

SHOULD EVERY COMPANY IMPLEMENT SIX SIGMA INITIATIVE

Six Sigma initiative is for those organisations which have a fire within “we want to be a quality leader” and possess the determination “to produce high quality products or to achieve error free processes”. It requires a high degree of commitment to quality on the part of top management.