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6 Sigma

6 Sigma is an improvement methodology developed to reduce variability in processes. Like Lean, Six Sigma is like a toolbox with many tools. Although a small part of the tools in it are also used in Lean, it mainly consists of Problem Definition, Problem Solving, Data Analysis and Statistical Methods. Six Sigma is mostly used to solve complex problems based on quality. For example, the fact that the diameters of some of the bolts you produce are not within the desired tolerances is a situation that can be easily solved with the Six Sigma Method.

What is Variability?

Variability is the fact that you do not consistently get the same results from the outputs (such as quality, efficiency) that you expect from the process. Basically, there are two types of variability in a process:


1.Left case: The case where the desired parameter is above and/or below the tolerances (or specification limits). Let’s call it the centering problem. For example, the process runs slower than desired, the drill bit drills above the set value, the fabric is darker than the color we want.

2. Right situation: The situation where the desired parameter is scattered over a larger area than desired. Let’s call this the variability problem. For example, the cycle rate for the same product varies at different times, the color tones are different in different meters of the fabric.

There may also be cases where both occur at the same time.

Variability is present in every process. Sources of variability may be due to known reasons (your journey home, which took 30 minutes on average, took 45 minutes due to snow) or the natural oscillation of the process (sometimes 30 minutes, sometimes 32 minutes).

For this reason, in the example below, tolerances (specification limits) have been established since it is known that the product desired to be produced as 30 microns cannot always be produced at this value.

When your process goes outside these limits, it produces erroneous output. The more you reduce process variability, the less likely you are to produce faulty output. This will increase customer satisfaction.

6. How to Solve Problems with Sigma?

With 6 Sigma, the inputs that cause these variations are determined and formulated as in the picture below.

Y=f(x) is used to define functions. At first, the problem/s (Y or ys) are defined. Then, the inputs that we think affect this problem are defined. (xs) Of course, since many x are defined, it is necessary to prioritize and reduce them to the amount we can control. For this, an approach called Logic Filters is used. Logic Filters have been developed in 6 Sigma to solve problems with 5 phases called DMAIC. Many inputs (xs) mentioned above are eliminated with different tools used in each phase and reduced to the controllable amount that affects the output the most.


What is DMAIC?

​DMAIC is the name given to 6 Sigma’s unique problem-solving management phases. These phases are called Define, Measure, Analyze, Improve, Control in English. DMAIC is the abbreviation of these phase names. Each phase is designed to determine the most important inputs (xs) that affect the output (Ys) by using the tools in itself. Many Operational Excellence problems can be solved if the steps are followed correctly.

We interpret the phases in Turkish as follows:

Definition: It is the phase where the output and potential inputs of the problem are defined, the scope is determined, and the project definition document is created.

Measurement: It is the phase in which the defined outputs and inputs are measured with accurate and sensitive measurement systems, and the accuracy and precision of the measurement results are ensured by analyzing the measurement systems (MSA).

Analysis: It is the phase in which the important factors that create the variability in the output are determined by graphical and statistical analyzes.

Improvement: It is the phase in which the important factors affecting the output are improved and optimized.

Control: It is the phase where actions are taken to ensure the continuity of the improved process.

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