The Difference Between Lean and Six Sigma
Lean is elimination of waste. Six-Sigma is about elimination of variation. Its goal is to produce output that is consistent and free of defects. Defective output requires re-work and re-work is waste. Inconsistent output is the result of a flawed process. Six-Sigma seeks to identify the flaws, determine their cause, and eliminate them.
Six Sigma DMAIC starts with the view that waste is defective products, to remove waste you find the root cause of defects (using statistical methods) then design an improvement (Design for Six Sigma) or replace a fault so that the defects no longer occur.
Using a manufacturing example again, let’s say you have a factory that produces widgets. The widgets are all supposed to be a certain size, shape, and weight. You sample your output and discover that out of every 1,000 widgets fifty are flawed (e.g. wrong size, shape, or weight). To correct this you could use Six-Sigma to determine what’s causing the variation/defects and work to reduce them to a six-sigma level.
Lean must begin with the view that the product and process that creates it must add value to the customer. Anything that does not directly add value to the customer is a waste. It further goes on to define the common wastes and how to eliminate them.
Lean vs Six Sigma
While it may be desirable to use both lean and Six-Sigma, they are separate issues. An operation can be lean and still have an unacceptable level of variation in the output. It can also have the variation under control and not be lean. The two are complimentary, but neither is dependent on the other.
Which to do first? Lean or Six Sigma?
Undoubtedly in an ideal world you should do Six Sigma before lean.
Why should you do six sigma before lean? Because lean is about creating an efficient process and six sigma is about making the process deliver what the customer wants with little variation. You don’t want to do lean first because you could make a perfectly-efficient, process that delivers something that the customer doesn’t want. If something doesn’t need to be done in the first place, there is no point in doing it well.
If you do Six Sigma first, you’ll have a process that can reliably produce output that the client wants repeatedly. Then, use Lean to make that process very efficient and productive.
In other words, use Six Sigma when your process has a lot of variation or doesn’t always perform how the client wants it to. Use Lean when your process already delivers what the customer wants but the process itself needs to be more efficient or more productive.
Both are tools, philosophies and management systems but geared towards different ends.
It is very possible that both processes could use the same or similar tools and end up with the same improvements.
Note that in the real world some management teams are more focused on efficiency and productivity than customer satisfaction, and in those cases Lean is the best choice. Also, it is sometimes helpful to do quick Lean activities so you can show improvements quickly (whether or not the customer cares about them) so you can build credibility to do the longer Six Sigma projects.
Tools Used by Both Six Sigma and Lean
In both cases process mapping and value stream mapping are essential. If you cannot define your process, how can you possibly know what is wasteful or what is variation? Once you have defined your process via a process map you can use value stream analysis to determine where the waste is.