Process Variation – the Enemy!

photo credit: kevindooley

The most important goal of understanding the principle of natural process variation is to consider the natural variance in the output before we make any changes to the process. Since SPC tends to minimize the process variations in time, as we better understand the process and have more experience with running it, we try to reduce the variation of it. The knowledge of the principle of natural variance helps us avoid making any unnecessary changes to the process, which might add variance to the process, instead of removing it. Warren Brussee in Statistics for Six Sigma: Made Easy! shows how Six Sigma can affect process variation:

The technical goal of the Six Sigma methodology is to reduce process variation such that the amount of unacceptable product is not more that 3 defects per million parts.

In Figure 1 below, three pictures help explain the variation in a process. The picture on the left displays a process that covers the entire target. While all the bullets appear to have hit the target, very few are in the bullseye. This is an example of a process that is centred around the target, but very seldom meets the CTQs of the customer.

The middle picture in Figure 1 displays a process that is well grouped on the target (all the bullets hit the target in close proximity to each other), but is well off target. In this picture – like in the first picture – almost every service or product produced fails to meet the customer CTQs.

The far right picture in Figure 1 displays a process that is well grouped on the target, and all the bullets are within the bullseye. This case displays a process that is centred and is within the tolerance of the customer CTQs. Because this definition of conformance defines “good quality” with all of the bullets landing within the bullseye tolerance band, there is little interest in whether the bullets are exactly centred. For the most part, variation (or dispersion) within the CTQ specification limits is not an issue for the customer.

Statistical thinking provides practitioners with the means to view these processes holistically. The table right at the bottom of this post reflects the Six Sigma tools that map to these principles, which are not exhaustive in nature and could overlap with other principles.

How to attack variation

Collins, in the book Good to Great (quoted in Six Sigma for Dummies (For Dummies)) , makes the point that:

greatness is not a function of larger-than-life leaders, exorbitant executive compensation, killer business strategies, advanced technologies, mergers and acquisitions, or big change technologies.

According to Collins it is disciplined people, thought, and actions over an extended period:

the right people must be on the bus to confront the most brutal facts of your current reality

This point is backed-up by Thomsett in Getting Started in Six Sigma, who has suggested that:

most quality programs fail after a while because employees lose faith in these programs. It becomes obvious to employees that quality control really represents management’s attempts to cut costs and expenses and to get more work from the labour force.

So in summary I believe that in Six Sigma a Black Belt’s skills in maintaining and assisting team members toward self-actualization, and in monitoring and addressing key individual performance factors is critical to overall team success. He/she must also understand process variation and be able to communicate this to a well-maintained team.

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Comments

By yenan on November 17th, 2006 at 3:20 pm

thanks Robert for your articles…
p.s. just don’t understand why such a quality quality blog has only 9 rss readers up to now…

By yenan on November 17th, 2006 at 10:20 am

thanks Robert for your articles…
p.s. just don't understand why such a quality quality blog has only 9 rss readers up to now…

By robert on November 17th, 2006 at 3:23 pm

yenan – thanks for the nice words. Perhaps six sigma only appeals to a small majority of people? Eveyone “varies” in their interest – an example of process variation in action I think!

By robert on November 17th, 2006 at 10:23 am

yenan – thanks for the nice words. Perhaps six sigma only appeals to a small majority of people? Eveyone “varies” in their interest – an example of process variation in action I think!

By Anonymous on December 5th, 2006 at 6:49 am

Robert,

I’ve recently stumbled across your blog, and while I think it’s interesting I’ve noticed references to other’s articles on your site without proper attribution. For instance, there is one graphic in the article I’m making my post that came from one of the article I published on iSixSigma.com (http://www.isixsigma.com/library/content/c030120a.asp) back in 2003. Surely someone with your education would know to give a simple reference for the graphic so as not to take other’s work and portray it as your own. It happens all the time and I’m sure if simply an oversight. But I do a lot of reading and think I’ve read phrases from some of my other friends and colleagues. A lot of this stuff runs together. My only request is that you give reference to thought of not your origination. Having said that, no one sees farther than standing on the shoulders of giants. Keep up the good thought. Charles Waxer

By Anonymous on December 5th, 2006 at 1:49 am

Robert,

I've recently stumbled across your blog, and while I think it's interesting I've noticed references to other's articles on your site without proper attribution. For instance, there is one graphic in the article I'm making my post that came from one of the article I published on iSixSigma.com (http://www.isixsigma.com/library/content/c03012...) back in 2003. Surely someone with your education would know to give a simple reference for the graphic so as not to take other's work and portray it as your own. It happens all the time and I'm sure if simply an oversight. But I do a lot of reading and think I've read phrases from some of my other friends and colleagues. A lot of this stuff runs together. My only request is that you give reference to thought of not your origination. Having said that, no one sees farther than standing on the shoulders of giants. Keep up the good thought. Charles Waxer

By robert on December 5th, 2006 at 7:00 am

Charles – many thanks for the comment. I do try to cite my references however, I have a lot of material stored offline for later peronal reading and sometimes I simply lose track of where it came from. I completely acknowledge that this graphic came from the article you mention and will of course remove it if you request. It is my policy to always attempt to allocate proper credit, however, as I’ve said I don’t always know who that actually is unless its pointed out. I’ll certainly work on a system for maintaining this from now on to make sure it never happens again: firefox scrapbook entension looks promising?

By robert on December 5th, 2006 at 2:00 am

Charles – many thanks for the comment. I do try to cite my references however, I have a lot of material stored offline for later peronal reading and sometimes I simply lose track of where it came from. I completely acknowledge that this graphic came from the article you mention and will of course remove it if you request. It is my policy to always attempt to allocate proper credit, however, as I've said I don't always know who that actually is unless its pointed out. I'll certainly work on a system for maintaining this from now on to make sure it never happens again: firefox scrapbook entension looks promising?