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Six Sigma in Medical Device Manufacturing

3 August, 2022

Six Sigma is a very common manufacturing strategy that is utilised by a large number of manufacturers across various industries. Here at Advant Medical, Six Sigma is one of the methods we use to nurture a lean philosophy and provide innovative manufacturing techniques. We consistently incorporate lean principles that maximise efficiencies, eliminate waste and streamline business processes to effectively increase speed to market for our customers. This article will explain what six sigma is, its origins and its benefits for medical device manufacturing.

What is Six Sigma?

In short, Six Sigma is a set of techniques used to improve a process by reducing variation and increasing process control. According to ASQ, Six Sigma is “a method that provides organisations tools to improve the capability of their business processes. This increase in performance and decrease in process variation helps lead to defect reduction and improvement in profits, employee morale, and quality of products or services.”

History of Six Sigma

Motorola was the first company to bring Six Sigma into the mainstream. They used the methodology to create more consistent quality in the company’s products. The system is different from but influenced by other management improvement strategies of the time, including Total Quality Management and Zero Defects. In 1995 this concept was developed by Jack Welch as the main concept of the business strategy of GE (General Electric) and is now used by several industrial sectors.

So, does it work? Well, Motorola reported in 2006 that the company had saved $16 billion using Six Sigma and almost half of the Fortune 500 companies had adopted this process for their organisations by the end of the twentieth century.

Six Sigma Concept

Normal Distribution

The below graph represents normal distribution and shows the assumptions underlying the Six Sigma model. The horizontal axis showing distance from the mean, marked in units of standard deviation or Sigma (σ). The upper and lower specification limits (USL and LSL) are at a distance of 6σ from the mean. Normal distribution means that values far away from the mean are extremely unlikely, and the same too high. If the process variation is controlled to six sigma levels the defects expected is 3.4 parts per million opportunities, even if there is a shift in the process mean by 1.5 sigma.

Image courtesy of Lean Six Sigma Definition.

Standard Deviation

The word Sigma comes from the Greek symbol “sigma” or “σ”, a statistical term that refers to the standard deviation of a process about its mean. A Sigma symbolises a single deviation. Six Sigma is the highest level of Sigma, which dictates that the margin of error can be up to six standard deviations from the mean. Six Sigma is an essential metric for measuring and controlling product quality.

How to calculate standard deviation:

  1. Find the mean: Calculate the mean of the data set.
  2. Find each score’s deviation from the mean: Subtract the mean from each value in the data set.
  3. Square each deviation from the mean: Square the differences found in step 2
  4. Find the sum of squares: Add up the squared differences found in step 3
  5. Find the variance: Divide the total from step 4 by either N (for population data) or (n – 1) for sample data. This is the variance.
  6. Find the square root of the variance: Take the square root of the result from step 5 to get the standard deviation.

Image courtesy of:

For example, let’s say we measured 6 parts and the measurements were 0.256″, 0.246″, 0.251″, 0.258″, 0.245″ and 0.249″. Using the above steps, we can calculate the standard deviation:

  • Find the mean: The mean of the data set is 0.2508333″
  • Find each score’s deviation from the mean:
  • 0.256″-0.2508333″=0.0051667″
  • 0.246″-0.2508333″=-0.0048333″
  • 0.251″-0.2508333″=0.0001667″
  • 0.258″-0.2508333″=0.0071667″
  • 0.245″-0.2508333″=-0.0058333″
  • 0.249″-0.2508333″=-0.0018333″
  • Square each deviation from the mean:
  • 0.0051667″^2=0.000027″
  • -0.0048333″^2=0.000023″
  • 0.0001667″^2=0.0000000028″
  • 0.0071667″^2= 0.000051″
  • -0.0058333″^2= 0.000034″
  • -0.0018333″^2=0.0000035″
  • Find the sum of squares: 0.0001385″
  • Find the variance: 0.0001385″/(6-1) = 2.7766″
  • Find the square root of the variance: 0.00526941
  • Standard Deviation= 0.005″

So, if the Standard deviation of a part is equal to 0.005”, then we can say that each score deviates from the mean by 0.005” on average. This 0.005” measure is 1 sigma.

Process Capability

Process Capability, the process and its capability, essentially means how capable the process is of satisfying the customer or whether the process is capable enough. The natural variation of a process should be small enough to produce products that meet the standards required. Processes can be influenced by many factors which cause deviations. To check the process capability, we must perform the process capability test. The below formula is how process capability is calculated:

Image courtesy of

A capable process must have a Cp of at least 1.0 and Six Sigma quality requires a Cp of 2.0.

For example, if a customer needs a part width to be 0.25” +/- 0.03, then the Upper Specification Limit (USL) is 0.28” and Lower Specification Limit (LSL) is 0.22”. These can also sometimes be referred to as specs or tolerances. Using the above example, let’s say the standard deviation is for this part is 0.005”. Using the USL, LSL and standard deviation we can calculate the Process Capability below:

Cp= (USL – LSL) / 6(σ)
Cp = 0.28-0.22 / 6(0.005)
Cp= 0.06 / 0.03
Cp= 2

Six Sigma Levels

This is a guide that is used to measure how often the defects will probably occur. A value from 1 to 6 signifies the maximum number of defects per million. This is why Six Sigma is the target, as it greatly reduces the number of defects in production.

Image courtesy of Research Gate.

Importance of Six Sigma in Manufacturing

Let’s use a scenario to explain how significant six sigma can be. Let’s say a production line when it first started had one reject in every 10,000 units, but now 1 year later it has 10 rejects in every 10,000 units. So where is the problem? This is where six sigma can help.

Joseph M. Juran, the quality management guru, said “changes creep up on us week by week, a little bit at a time. Over a year or two, there are 50 or 100 of these bits, which amounts to quite a bit. The skills of the men have not necessarily kept pace, and we wake up to the existence of a wide gap.”

Over time inefficiencies creep in and six sigma can help to trace back and identify their root cause. Perhaps skilled team members have left, and new members have not been sufficiently trained on how to make the units. Or maybe there has been a supply chain issue, and the quality of incoming materials is inferior, or it may be that the equipment used to manufacture the unit has deteriorated. Here, the Six Sigma method can be used to identify the bits of change so that management can decide on what needs to be corrected. The method follows a defined process, where the entire organisation looks at everything that it does in the light of reducing variation.

Different Methodologies of Six Sigma

There are many methods used within Six Sigma but two of the major ones are DMAIC and DMADV. DMAIC stands for define, measure, analyse, improve, and control. DMADV stands for define, measure, analyse, design, validate. As is evident, the first three stages are the same and the frameworks are also similar in that they use statistical data to identify quality-related challenges. The two methodologies often confuse, and it’s imperative to know what works where. According to Purdue the differences between the two methodologies is as below:

  • DMAIC addresses the current process; DMADV addresses the design process.
  • DMAIC reduces/eliminates defects (reactive); DMADV prevents defects (proactive).
  • DMAIC includes specific solutions; DMADV is part of the solution design process.
  • DMAIC includes controls to sustain the gains; DMADV includes verification and validation of the finished design.

The below flow chart is designed to assist with the process of selecting the appropriate Six Sigma roadmap:

Image courtesy of Key Differences.

As can be seen from the above chart, the processes are similar but used for different purposes. In general, DMADV is used with new services and product design and its use is limited for existing products and processes. When there is no existing product, DMADV can be implemented to design the product or process. DMAIC is used on a product or process that already exists but is no longer meeting customer needs and/or specifications or requires a potential improvement.

Below is a list of some of the other methodologies most commonly used within Six Sigma:

  • IDOV: Identity, Design, Optimise and Verify.
  • DDICA: Design, Develop, Initialise, Control and Allocate.
  • DMADOV: Define, Measure, Analyse, Design, Optimise and Verify.

Benefits of Six Sigma

1. Improve Accuracy and Control

Six Sigma measures a process’ accuracy as it aims to prevent the occurrence of defects. Using Six Sigma techniques, you are given the chance to identify problem areas of these recurring defects so that you can address the root cause of the problem and find the right solutions. Variation will be eliminated when a standard becomes uniform for the company for a certain business process. Using Six Sigma can help this happen and it will in turn strengthen your quality and consistency. For example, it could be that there is a faulty device that is causing a defect and the solution would be to remove that machine from the process or fix it.

2. Improving Time Management

Every minute counts. Six sigma implementation helps to improve the overall efficiency of the process. It has also been proven to help employees better manage their time, enhancing productivity. If the focus area of the six-sigma project is to save time then the company will likely see improvements to timelines in delivering the process output or improving the on-time delivery of products or services every time.

3. Reducing Waste

Implementing Six Sigma helps to reduce waste from a process. Waste is defined as anything that doesn’t help in producing the service or product that is required to be delivered to a customer. Reducing any wasteful activity facilitates continual improvement of the process which adds value to the customer.

4. Increase Customer Loyalty/Service

There is a lot of choice in every market, so you want to stand out, but for the right reasons. to distinguish themselves from competitors by providing quality in offered products and services at comparable prices. Consequently, Six Sigma has proven to be necessary for achieving the ultimate goal of increasing customer loyalty and satisfaction. Six Sigma projects can help identify the number of variations customers are experiencing, what is driving that variation, and ultimately, how the number of dissatisfied customers can be minimised. Such Six Sigma projects can also be aimed at improving customer & vendor satisfaction.

5. Higher revenues and lower costs

Every minute counts. Six sigma implementation helps to improve the overall efficiency of the process. It has also been proven to help employees better manage their time, enhancing productivity. If the focus area of the six-sigma project is to save time then the company will likely see improvements to timelines in delivering the process output or improving the on-time delivery of products or services every time.


As with any methodology, Six Sigma does have some drawbacks, some of which have been highlighted below:

  • ● The Six Sigma approach creates a lot of data. While data can be used to help benefit the company, dissecting the business process minute-by-minute creates a lot of empirical data which leads to time-consuming and complicated procedures.
  • ● As Six Sigma is applied to all aspects of the production and planning process, it may create rigidity that can create delays and stifle creativity.
  • ● As it is a quality improvement process at its root, adoption of protocols often leads to an increase in costs
  • ● Team members can become too focused on Six Sigma. For example, a more expensive material that helps to achieve six sigma may be chosen over a cheaper material that carries a slightly higher defect rate. While this helps the company to achieve six sigma, it will adversely affect its profitability.


The Six Sigma methodology, particularly when partnered with Lean Manufacturing – Lean Six Sigma – maximises an organisation’s potential, increases team morale and effectiveness, and minimises waste and downtime. Using DMAIC and other methodologies will provide the roadmap for your improvement journey. However, change won’t happen overnight. It took GE three years to “get it”. Yes, there will be early successes but it might be 5 years before the transformation happens throughout the organisation. Six Sigma works. The key is whether you and your organisation can embrace it and make it work for you.

Advant Medical

Since 1993 Advant Medical has become the Global ‘Partner of Choice’ for Class I, II, III medical device development, injection moulding, contract manufacturing and packaging solutions. We have evolved to become a strategic manufacturing ‘Partner of Choice’ for a wide range of clients such as entrepreneurs, engineering and development groups, new start-ups, SME medical device companies & multinationals. We are specialists in minimally invasive access & delivery devices providing a full range of services required to deliver finished medical devices to market. Talk to us today to learn how we can partner with you on your next project here.

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