Introduction to Six Sigma

Six Sigma is a process improvement tool used to measure quality in order to reduce or eliminate defects. The thought behind Six Sigma is that it is a way of approaching a process that is disciplined, using data to eliminate defects and reduce the variation of these processes. How can your organization benefit from it? In many ways, but Six Sigma is a commitment to improving your whole organization, not just a few things here and there.

More specifically, while the above explanation of Six Sigma may seem simple, Six Sigma is actually fairly complicated because the process is entrenched in mathematics and statistical methodology. For example, the goal of Six Sigma is to produce a process where there are no more than 3.4 defects per million opportunities.

What is a defect and how would you define opportunities within Six Sigma? A defect is anything that is outside of the customer’s specification, while an opportunity is the chances for a defect to occur during the process. Let’s take a look at a simple calculation. Suppose you produce 100,000 gadgets in a week, and it is discovered that the defect rate is 15 defects per 100,000. When using a Sigma calculator, your results are:

DPMO: 150
Defects (%): 0.02
Yield (%): 99.99
Process Sigma: 5.12

What exactly do these numbers mean? Let’s break it down ever further so you can see what exactly you’re looking at. DPMO is “defects per million opportunities”. Since the goal of true Six Sigma is to only have 3.4 defects per million opportunities, looking at 150 defects doesn’t seem like that is too good.

However, there is more that goes into determining the amount of defects in order to have truly implemented Six Sigma, such as knowing if there is one specification limit or two. This affects the Upper and Lower standard deviations between the customer specifications and the process. The Process Sigma is 5.12, which as it increases from zero to six, the variation in the process around the mean value decreases. As the value of the process sigma increases, the variation decreases creating a process with zero defects.

Six Sigma literally means the number of standard deviations away from the mean, or the average, as indicated on a bell curve. This is also known as the normal distribution. Thus when calculating the mean, Six Sigma allows for 3.4 defects per million. In essence, if you are making one million widgets, then you should only have 3.4 defects per that million widgets produced. The rest of the widgets fall under the “normal distribution” as indicated on the bell curve. Below is an example of what the bell curve looks like—indicating the deviation from the mean, which is Zero, or the target.

When looking at the graph above, LSL is the Lower specification limit, and the USL is the upper specification limit. Again, Six Sigma allows for six process standard deviations between the mean in the process and what the customer’s specification limit. As your value process stigma increases from zero to six, then the variation in your process around the mean will decrease. This indicates that if your value of process sigma is high enough then when the process reaches zero variation you have reached zero defects.

Six Sigma quality is not easy to achieve, particularly when you roll throughput yield into the mix, resulting in yields of each process is multiplied together to obtain the final yield. In short, this is the percentage of good widgets that are produced in a given process. For instance, if there are four different processes, each having a four percent (4%) yield the total throughput yield is calculated as: .99 x .99 x .99 x .99 = 96%. This is a great example of how Six Sigma can work and how when the proper check points are established within each process, no defects are passed onto the next process or stage within production.

This is a lot to process at this basic level, but in essence, Six Sigma and the role of the professional is to quantify the process performance, which is the short and long term capability, taking the process entitlement and process shift, to create the right strategy in order to reach the determined performance objective. One thing to remember when determining what your short term and long term capability is, that when you decrease your process variation, your process sigma increases, resulting in greater customer satisfaction and lower costs.

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