What is the difference between Lean and Six Sigma?

Lean whiteboard example design mockup

Lean and Six Sigma have been around for a long time now, so it’s perhaps foolish to think that some people don’t know what they are, what the difference is, and how they can be used together. But it seems that this is indeed the case – it feels like I am explaining them one way or another every month or so to a project team, or to a new process improvement team, or to a new change team. A recent request to explain how they differ inspired this post.

So, apologies in advance if you already know this…

Describing Lean and Six Sigma

One way I like to describe them is that one, Six Sigma, is an American top-down method of management control. The other, Lean, is a Japanese bottom-up production management method. I usually describe them this way if I’m feeling rude or mean about Lean Six Sigma, which I will be, just a little bit, later in this post.

Generally, they are explained the following way – they’re both process improvement methods, but Lean concentrates on eliminating waste, and Six Sigma concentrates on eliminating defects and variation. As an explanation, it is not without truth. But, I think it misses a bigger point about their shared emphasis on the process’s customer and the deliberate listening to that voice by the improvement team. One key similarity is that they both focus on the current process, the as-is process. So they are both different from more radical methods like business process re-engineering, which focus on the future process (and often start with a blank sheet of paper to do so).

Six Sigma, standard deviation and the defect scale

So, the key points about Six Sigma start with the name. The Greek character sigma (σ) is used in statistics to denote standard deviation, which is a measure of how varied a set of data is. So, consider two football clubs, both with a squad of 40 players, and an average wage of £55,000 per week. If we know that the standard deviation of City’s wages is £4,000 and of United’s is £7,000, we can conclude that the range of salaries United pay is more varied than that of City. United probably has more players at either end of the range than City (this is, of course, a made-up example…).

This wage variation might not matter much in a football club. However, variation in a manufacturing or customer service process could be critical, as the outliers in the range typically represent unacceptable defects outside the tolerance for the process. In Six Sigma we measure the number of defects in a process as a percentage, and use a scale based originally on standard deviations to calculate the sigma score (symbol: Z) of a distribution. The scale is shown below.

The Sigma scale
So, a process at six sigma will only fail 3.4 times in a million executions. By one measure, that’s about 70 times better than a process at five sigma. Six Sigma projects find the current Z score and set new targets for an improved score. Hopefully, this links to the actual quality requirements of the business.

Process improvement measurement with Six Sigma

Six Sigma projects use a standard approach:

• Define
• Measure
• Analyse
• Improve
• Control

usually referred to by its acronym DMAIC. In a nutshell, find out how many defects there are, analyse what is causing them, eliminate the cause, re-measure. Wash and repeat.

Six Sigma is all about statistical process control (SPC), and control charts are a major visual output of the method. It is heavy on statistics. My own view of it is that the process improvement activity is often largely obscured by, and subsumed into, the process measurement and analysis activity. But it is rich in tools and techniques for those prepared to stay the course.

Lean as a production management system

Lean is a production management system developed originally by Toyota, with its origins in the 1920s. It was refined over some four decades, reaching maturity in the 1950s. They knew it as Toyota Production System (TPS) We now know it as Lean, a name that was given to it in the 1980s by an American team studying Toyota and similar companies (Lean Thinking by Womack and Jones, the leaders of this team, was published in 1996).

For me, the key concepts in Lean are:

• Waste – if it doesn’t add value, it’s a waste. Lean defines eight types of waste (there were seven originally). A lot of process analysis is focused on identifying, process step by process step, these wastes, and removing them. (Some process steps are non-value adding (to the customer), but necessary for regulatory or legal reasons.)
• Pull (as opposed to Push) – in a Push process, Step 1 controls the execution of Step 2. In a Pull process, Step 2 controls Step 1. Think of a supermarket – in a Push system, products are created (step 1) and then put on shelves (step 2). This can be a problem if the products already there haven’t been shifted. In a Pull system, the space occupied by the products is monitored. At a predetermined level, a signal is sent back to the production process to make some more, in the full knowledge that there is space on the shelf for them, and they are likely to be sold. In this way, the products are “pulled” through the system.
• Flow – in Lean, the production flow (the “value stream”) is designed not to stop, once started. This requires a comprehensive understanding of value as perceived by the customer, and of the production process.

Lean Value Stream Map

One of the major visual outputs of Lean is the Value Stream Map of the process. It is this which is used to calculate the flow metrics, to keep the whole thing moving. The diagram below is an excellent example; it was produced by Xendo, a Dutch firm now part of ProPharma. (It is used by kind permission of Marc Stegman of ProPharma.)

Example Value Stream Map from Xendo

Lean is much more than its focus on waste. It is the focus on customer value which (if applied properly) gives it its power. Its wide range of analytical and production management techniques is impressive, although significant improvements can be achieved with a focus on just the essential elements of Lean.

Integration of Lean and Six Sigma

So, what is Lean Six Sigma? It is an attempt to integrate the two methods. My own view, which probably appears cynical, but is based on my observation of both camps and their go-betweens, is that:

• Some Six Sigma people have noticed the effectiveness of Lean’s “pull” techniques, and its waste classifications, and slotted them into the Improve step.
• Some Lean people have had a crack at some of Six Sigma’s statistics, but most give up and rely instead on an empirical and observable improvement in process performance. Some Lean people also get a bit sniffy about Six Sigma’s top-down control culture, and prefer to walk around talking cod Japanese.
• Both Lean and Six Sigma have some cult-like attractions for their adherents, although this is much more pronounced on the Lean side, than it is on the more arid Six Sigma side.

Conclusion – take a project specific approach

Most mature process improvement practitioners will recognise that the method needs to be driven by the problem, not the other way round. So most of us are prone to borrowing the things that we know work in a variety of methods, and combining them into an approach for a specific project. This is not quite the same as creating a new methodology…

No-one interested in process improvement should be ignorant of either method, and some immersion in one or the other will be repaid. For me, Lean would be my starting point, but I would spend some time getting to grips with the sigma scoring method in Six Sigma. There, it looks like I’ve just (accidentally) recommended Lean Six Sigma…

A piece of trivia: an early version of Pink Floyd was called Sigma Six (partly because it was a sextet (occasionally a septet), but I am still trying to find out the origin of the “Sigma”). A later incarnation was called The Spectrum Five – I keep looking out for a management method called Five Spectrum – haven’t found it yet. (Later, of course, both Sigma and Spectrum were used as brand names for computers…)

Next steps

If you would like to learn more, we offer training workshops in Lean, Six Sigma and other popular visual management systems. We can design workshops to introduce a system across your organisation, or to bring new starters up to speed.

We can also help you communicate your visual outputs more effectively with a bespoke toolkit to include whiteboards, magnetic overlays and document holders, and printed and shaped magnets (e.g. Value Stream Map or Ishikawa/fishbone diagram, part of the Analyse stage of DMAIC).

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