Six sigma

Six sigma ($6\sigma$) is a method of production system improvement performed in order to achieve a product of perfect quality. The method base on statistical process control and other statistical tools to find causes of problems. The team of employees works on causes to remove it. The method was developed by Motorola in USA in 80s and popularized after the company achieved Malcolm Baldridge National Quality Award.

The name of the method refers to statistical tools. The 6 sigma is the level on which only 3,4 defects occurs per 1 million of operations (not products! each product requires many operations). That means 99.9997% error free operations.

Nowadays Six sigma is often linked with lean approach as Lean-six sigma.

Application of Six sigma

The statistics behind six sigma is really simple (see below). The heart of the program is organizational change. It is relatively easy to improve quality to 2-3 sigma buying new equipment. But each next level becomes harder to achieve. In literature the level of 4.7 sigma is known as critical. Moving beyond this requires usually rethinking of all the process. Organizations don't have to reach 6 sigma with each operation. They usually go so far with the most important ones.

DMAIC improvement cycle

DMAIC is modified PDCA cycle, which includes:

• Define the goals
• Measure characteristics that are critical to quality
• Analyse the process and product and develop alternatives
• Improve the product and process
• Control the future state of the process (to avoid entropy)

Some authors propose modified names of the cycle.

Black belts, green belts

Black belts and green belts are managers or employees that have certain roles in the program. The main roles are:

• Leaders - the CEO and top management
• Champions - upper management, mentors of black belts
• Master black belts - in-house coaches
• Black belts - operate under masters in specific projects
• Green belts - employees who work with six sigma implementation

The system of roles shows that program is being implemented with extensive participation of higher management. This is the opposite approach to Japanese ideas like Total Quality Management or Lean management. It's one of the weaknesses of Six sigma.

Methods

The Six sigma program uses many well known methods, e.g. Quality Function Deployment, Quality loss function, Fish diagram, Root cause analysis, 5 whys.

Statistics behind Six sigma

Example of normal distribution. Mean, standard deviations and control lines are shown. Many products are behind control lines - quality can be improved (not to scale)

To understand what's behind Six sigma we need several terms:

• normal distribution (the Gauss curve)

Variability of parameters is typical for every process. After many repeats of the process, we can collect enough data to draw a histogram showing what values monitored parameters have. Let's assume, you cut slices of bread. Each will have a bit different thickness. If you measure the thickness and draw a histogram you'll probably end with a bell curve. The most of slices will have thickness about the mean, and the further from the mean the smaller the number of slices.

• standard deviation ($\sigma$)

It's a measure of variability. You can compute it analysing difference between mean thickness and thickness of each slice of bread. The standard deviation is interesting, because:

• between mean - $1\sigma$ and mean + $1\sigma$ there should be 68.26% of all data.
• If you increase this scope to mean plus/minus $2\sigma$ it will be 95.44%.
• In case of mean plus/minus $3\sigma$ it will be 99,74%.
• In case of mean plus/minus $6\sigma$ it will be as you already know 99.9997%

• control chart

The control chart was invented by Walter A. Shewhart. It shows the variability of the process on simple chart. It defines two limits: lower control line and upper control line. It the parameters of the product are within those lines - the product is good, otherwise it's a defect.

The same process after 6 sigma improvement. The standard deviation is so low, that 6 sigma fits between mean and the control line (not to scale)

We want all the products to be within control lines. The idea of Six sigma is to make the process very stable. This allows to reduce the standard deviation. It should be so small, that 6 standard deviations from the mean should fit between mean and each control line. In that case our we'll achieve 99.9997% correct operations.

Criticism

Modern research results show that 6 sigma programs only in much less than 50% of companies end with profits. The success of Motorola was not repeated by over half of enterprises that tried to do it.

Examples of Six sigma

• Process improvement in a manufacturing company: A manufacturing company used the Six Sigma method to identify areas of improvement, such as reducing the number of defects in their products and increasing efficiency in the production process. By implementing changes suggested through the Six Sigma process, the company was able to increase its overall profitability and customer satisfaction.
• Quality improvement in customer service: A customer service department implemented the Six Sigma method to identify areas of improvement. The team was able to identify common customer complaints and measure the effectiveness of the department’s response to each one. Through the use of statistical data, the team was able to develop a plan to improve customer satisfaction and experience.
• Reduction of waste in business operations: A business used the Six Sigma method to identify areas of waste within their operations. The team was able to identify areas where resources were being used inefficiently and develop a plan to reduce the amount of waste generated. This allowed the business to become more efficient and profitable.

Advantages of Six sigma

Six Sigma is an effective method for improving production systems, as it provides a set of tools and processes that can help identify and remove causes of problems in order to achieve a product of perfect quality. The main advantages of Six Sigma include:

• Improved Quality: Six Sigma helps to reduce variation in processes, resulting in improved quality, higher customer satisfaction and reduced costs.
• Improved Customer Satisfaction: By reducing defects and improving quality, Six Sigma helps to increase customer satisfaction.
• Improved Process Efficiency: Six Sigma helps to identify and eliminate sources of waste, increasing process efficiency and reducing costs.
• Increased Productivity: Six Sigma helps to reduce costs and increase efficiency, resulting in increased productivity.
• Increased Profitability: By improving process efficiency and reducing costs, Six Sigma helps to increase profitability.

Limitations of Six sigma

Six Sigma is an effective production system improvement method, however, there are some limitations to it. These are:

• Lack of Flexibility: Six Sigma is a relatively rigid system as it requires very specific steps to be followed. This can be difficult to implement in a dynamic environment, where change is frequent and unpredictable.
• Cost of Implementation: The cost of implementing Six Sigma can be high, due to the training and certifications required for team members. Additionally, there is the cost of employing an expert to oversee the process.
• Time Intensive: As the Six Sigma process includes a number of steps, it can be time consuming to implement.
• Lack of Motivation: As Six Sigma focuses on improving existing processes and finding errors, it can be difficult to motivate team members to work on the process.
• Limited Scope: Six Sigma is limited to improving existing processes and does not address issues such as innovation or customer satisfaction.

Other approaches related to Six sigma

In addition to Six Sigma, there are several other approaches to production system improvement that can be employed to achieve a product of perfect quality. These include:

• Lean manufacturing: Lean manufacturing is an approach aimed at reducing waste or non-value-added activities in production processes. It involves streamlining processes and eliminating activities that do not add value to the end product.
• Total Quality Management (TQM): TQM is a systematic approach to quality improvement that seeks to maintain quality throughout the entire production process. It is based on the principle of customer satisfaction and involves all aspects of the production process.
• Kaizen: Kaizen is a Japanese term meaning "continuous improvement". It is an approach that seeks to continuously improve processes and products through employee involvement and small, incremental changes.
• Just-in-time (JIT): JIT is an inventory control system that seeks to eliminate unnecessary costs such as storage and transportation. It involves ordering and receiving materials only when they are needed to meet customer demand.

In summary, Six Sigma is just one of many approaches to production system improvement, with Lean manufacturing, Total Quality Management, Kaizen and Just-in-time being some of the other approaches. Each of these approaches has its own merits and can be used to help achieve a product of perfect quality.

Six sigma — recommended articles

Total quality control — Lean manufacturing — Philip B. Crosby — Deming wheel — ISO 9001 — World class manufacturing — Walter A. Shewhart — Quality control — Lean management

References

• The Six Sigma Handbook
• Kwak YH, Anban FT (2006) Benefits, obstacles, and future of six sigma approach, Technovation, 25:5-6
• Linderman K, Schroeder RG, Zaherer S, Choo AS (2003) Six Sigma: a goal-theoretic perspective, Journal of Operations Management, 21:2

Author: Slawomir Wawak