Special and Common Cause Variability, a Deming Management System
Dr. W. Edwards Demming – American Genius Part 1
W. Edwards Deming, in his book, Out of the Crisis (Massachusetts Institute of Technology, 1982) stated, “We shall speak of faults of the system as common causes of trouble, and faults from fleeting events as special causes….”
Does Deming and Shewhart view the theory of variation differently? Forrest Breyfogle says that mathematically the control limits for a Shewhart x-bar and R chart are not affected at all by variability between subgroups.
To illustrate the implication of this consider that you have a raw material lot that changes every day and it has a large affect on the response of the process. Remember we are subgrouping by day. The x-bar plot of an x-bar & R control chart will invariably show many out of control signals; i.e., the Shewart term “assignable cause” can be applied to the raw material.
However, most people I ask will consider that the natural up and down variability of the raw material inputs to be a potential source of common-cause variability (Deming term) to the process; i.e., it is just the variability from an input parameter.
If the mean value is analyzed using an individual control chart, this variability will show as common cause; i.e., data will be in control. This is because the variability calculating the control limits for the individuals control chart is the moving range between adjacent subgroups.
Additionally, if only a single sample were chosen daily and placed into an individuals control chart (I-Chart) (instead of using multiple samples in a subgroup), the data would be shown as “common cause” variability.
The enterprise can benefit most from Six Sigma when they have an integrated scorecards and strategic planning system which leads to targeted projects that have the most enterpise-as-a-whole benefit. Silo projects in isolation that appear beneficial but have little overall enterprise financial benefit can be avoided if this approach is used. For example, from a manufacturing point of view Six Sigma production efficiency-improvement projects could appear beneficial; however, if currently the organization has much excess capacity, it can be more advantageous, from an overall business perspective, to focus on improving sales and marketing efforts.
The Enterprise process DMAIC (E-DMAIC) business-system roadmap is a systematic means to identify and execute projects that have the most benefits to the business. Strategies created analytically and innovatively can establish goals for an organization’s value chain 30,000-foot-level reported metrics that would have the greatest dynamic impact on the financials using the Integrated Enterprise Excellence system. These improvement needs would “pull” or create improvement projects that woudl impact these metrics in a postive manner.
Organizations can enhance Lean Six Sigma Black Belt training for their six sigma black belts by including Forrest Breyfogle’s Integrated Enterprise Excellence System that goes the Balanced Scorecard.
Several 30,000-foot-level articles can be found in the “On-line Resource Library” link at www.SmarterSolutions.com.
The white paper, “Creation of Effective Organizational Predictive Metrics that Lead to the 3 Rs of Business” by Forrest W. Breyfogle, III describes on how to create 30,000-foot-level charts for various situations and much more. You can also visit www.ieeblackbelt.com for more information on lean six sigma black belts.