Six Sigma Quality Resources for Achieving Six Sigma Results
Click To Learn More About PremiumLinks
 Home > Statistics  > Variation Search:
 
 for    
Publications
Marketplace
| iSixSigma
Stuff
| iSixSigma
Blogosphere
| Events
Calendar
| The
Dictionary
| Discussion
Forum
| Find
a Job
| Post
a Job
| Industry
News
| Newsletter
Signup
| Sigma
Calculator
| Online
Surveys
DMAIC 2009 Training Slides: 1,220 PPT Slides + Instructor Notes and More for $99.95
iSixSigma Magazine Signup
 iSixSigma Live!  
  2010 Summit & Awards
  2010 Energy Forum
 Free Newsletters!  
  Sign Up Now!
  Manage Subscriptions
  New To Six Sigma?
  Six Sigma Q&A
  Cert. Practice Test
  Problem Solving Wizard
  ISSSP Info
ISSSP Is The Official Six Sigma Society of iSixSigma
 Channels 
  Europe
  Financial Services
  Healthcare
  Military
  Software / IT
 Quality Directory 
  Best Practices
  Certifications/Awards
  Consultants
  Culture Evolution
  Methodologies
  News & Events
  Organizations
  Product/Service Guides
  Statistics & Analysis
   Normality
   Variation
  Tools & Templates
  Voice of the Customer
  Free Whitepapers
 Related Topics 
  Innovation
  Outsourcing/Offshoring
  Business Process Mgt
 Quick Access 
  Help
  Search
  Advertise Here
  Article Archives
  Newsletter Archives
 User Feedback 
  Please suggest site
  improvements.
 
  [ larger form ]

Understanding Process Variation

Bookmark This Page Bookmark This Page
Email This Page Email This Page
Format for Printing Format for Printing
Cite This Article Cite This Article
Submit an Article Submit an Article
Six Sigma Article Archive Read More Articles
Related Tools & Articles
  • Six Sigma Quick Poll
    Does your business appreciate the concept of variation in improving processes?
    Yes
    No
    Not sure
    Discussion Forum
    "Generally short term variation is tested by taking consecutive samples in a short period of time. During this period operators are usually instructed to make no changes in the process. You therefore calculate the process variation (standard deviation) based on the best the process can do with out any changes. The long term variation usually collects data over time including process adjustments, many operators running the process...all the variation you might normally expect to be included in the process."
    Short Term Versus Long Term Variation
    Download Products
    By Charles Waxer

    It is well established that there exist 8 dimensions of quality:

    1. Conformance
    2. Performance
    3. Features
    4. Reliability
    5. Durability
    6. Serviceability
    7. Aesthetics, and
    8. Perceived Quality

    Each dimension can be explicitly defined and is self-exclusive from the other dimensions of quality. A customer may rate your service or product high in conformance, but low in reliability. Or they may view two dimensions to work in conjunction with eachother, such as durability and reliability.

    This article will discuss the dimension of conformance and how process variation should be interpretted. Process variation is important in the Six Sigma methodology, because the customer is always evaluating our services, products and processes to determine how well they are meeting their CTQs; in other words, how well they conform to the standards.

    Understanding Conformance
    Conformance can simply be defined as the degree to which your service or product meets the CTQs and predefined standards. For the purpose of this article, it should be noted that your organization's services and products are a funtion of your internal processes, as well as your supplier's processes. (We know that everything in business is a process, right?)

    Here are a few examples:

    1. You manufacture tires and the tread depth needs to be 5/8 inch plus or minus 0.05 inch.
    2. You approve loans and you promise a response to the customer within 24 business hours of receipt.
    3. You write code and your manager expects less than 5 bugs found over the life of the product per thousand lines of code written.
    4. You process invoices for healthcare services and your customers expect zero errors on their bills.

    A simple way to teach the concept of how well your service or product conforms to the CTQs is with a picture of a target. A target, like those used in archery or shooting, has a series of concentric circles that alternate color. In the center of the target is the bullseye. When services or products are developed by your organization, the bullseye is defined by CTQs, the parts are defined by dimensional standards, and the materials are defined by purity requirements. As we see from the four examples above, the conformance CTQs usually involve a target dimension (the exact center of the target), as well as a permissible range of variation (center yellow area).

    Figure 1: Targetting Process Variation
    Targetting Process Variation

    In Figure 1, 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 centered around the target, but very seldomly 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 eachother), 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 centered 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 centered. For the most part, variation (or dispersion) within the CTQ specification limits is not an issue for the customer.

    Relating The Bullseye To Frequency Curves
    In the real-world, we seldom view our processes as bullseyes (unless you work at a shooting range). So how can you determine if your process is scattered around the target, grouped well but off the bullseye, or grouped well on the bullseye? We can display our data in frequency distributions showing the number (percentage) of our process outputs having the indicated dimensions.

    Figure 2: Targetting Process Variation With The Process Capability Ratio
    Targetting Process Variation With The Process Capability Ratio

    One can easily see the direct relationship of Figure 2 to Figure 1. In Figure 2, the far left picture displays wide variation that is centered on the target. The middle picture shows little variation, but off target. And the far right picture displays little variation centered on the target. Shaded areas falling between the specification limits indicate process output dimensions meeting specifications; shaded areas falling either to the left of the lower specification limit or to the right of the upper specification limit indicate items falling outside specification limits.

    Interpretting Process Variation
    Most Black Belts have little time to completely understand the variation of their process before they move into the Improve phase of DMAIC. For instance, do the critical X's of your process have a larger impact on variation (spread) or central tendency (centering)? Segmentation or subgrouping the data can help you find the correct critical X. Hypothesis testing will help you prove that it is so.

    Conclusion
    Improvements in meeting customer CTQs and specification limits are objective measures of quality that translate directly into quality gains, because transactional processing errors, late deliveries and product defects are regarded as undesirable by all customers.

     
    Rate This Article:  Current Rating: 3.95
      Poor    Excellent     
              1    2    3     4    5
    Copyright � 2000-2009 iSixSigma – All Rights Reserved
    Reproduction Without Permission Is Strictly Prohibited – Copyright Requests


    Publish an Article: Do you have a Six Sigma tip, learning or case study?
    Share it with the largest community of Six Sigma professionals, and be recognized by your peers.
    It's a great way to promote your expertise and/or build your resume. Read more about submitting an article.




    "The Bottom Line" Links

    BEST SELLING PRODUCTS (iSixSigma Publications)
    1. Six Sigma Black Belt (DMAIC) Training Slides - 2009 Version!
      The 2009 Six Sigma Black Belt course includes over 40 more slides than the 2008 version. Contents include: 1,220 PowerPo...
    2. Certified Lean Six Sigma Black Belt Assessment Exam
      Interested in assessing your knowledge of Lean Six Sigma? Preparing for certifications? Testing your students and traine...
    3. Certified Lean Six Sigma Green Belt Assessment Exam
      This assessment exam is useful for students interested in assessing their knowledge of Lean Six Sigma on the Green Belt ...
    4. Kaizen Workshop E-book
      This 150+ page ebook teaches key tools and techniques of Kaizen, as well as real application to enhance learning. Kaizen...
    5. Certified Lean Six Sigma Black Belt E-book
      In 670 pages learn everything within the Lean Six Sigma DMAIC body of knowledge to successfully achieve Black Belt certi...
    6. Process Management Training Slides
      The 2008 Process Management course is designed in two phases comprised of:352 Powerpoint slidesInstructor notesSlide exp...
    7. Design For Six Sigma (DFSS) E-Book or Print
      Need an "encyclopedia" consisting of many of the tools you’ll study? Need a helpful refresher to apply the DFSS process?...
     
    Six Sigma AdLinks
    AdLinks Information


    Google AdWords
     
    Home | Discussion Forum | Event Calendar | Job Shop
    Link To iSixSigma | Rate This Page | Report A Problem | Free Content For Your Site | Submit Article For Publishing
     Terms of Service. �2000-2009 iSixSigma. All rights reserved. v3.0lb, 0.1
    About iSixSigmaContact UsPrivacy PolicySite Map