Accendo Reliability

Your Reliability Engineering Professional Development Site

  • Home
  • About
    • Contributors
  • Reliability.fm
    • Speaking Of Reliability
    • Rooted in Reliability: The Plant Performance Podcast
    • Quality during Design
    • Way of the Quality Warrior
    • Critical Talks
    • Dare to Know
    • Maintenance Disrupted
    • Metal Conversations
    • The Leadership Connection
    • Practical Reliability Podcast
    • Reliability Matters
    • Reliability it Matters
    • Maintenance Mavericks Podcast
    • Women in Maintenance
    • Accendo Reliability Webinar Series
  • Articles
    • CRE Preparation Notes
    • on Leadership & Career
      • Advanced Engineering Culture
      • Engineering Leadership
      • Managing in the 2000s
      • Product Development and Process Improvement
    • on Maintenance Reliability
      • Aasan Asset Management
      • AI & Predictive Maintenance
      • Asset Management in the Mining Industry
      • CMMS and Reliability
      • Conscious Asset
      • EAM & CMMS
      • Everyday RCM
      • History of Maintenance Management
      • Life Cycle Asset Management
      • Maintenance and Reliability
      • Maintenance Management
      • Plant Maintenance
      • Process Plant Reliability Engineering
      • ReliabilityXperience
      • RCM Blitz®
      • Rob’s Reliability Project
      • The Intelligent Transformer Blog
      • The People Side of Maintenance
      • The Reliability Mindset
    • on Product Reliability
      • Accelerated Reliability
      • Achieving the Benefits of Reliability
      • Apex Ridge
      • Metals Engineering and Product Reliability
      • Musings on Reliability and Maintenance Topics
      • Product Validation
      • Reliability Engineering Insights
      • Reliability in Emerging Technology
    • on Risk & Safety
      • CERM® Risk Insights
      • Equipment Risk and Reliability in Downhole Applications
      • Operational Risk Process Safety
    • on Systems Thinking
      • Communicating with FINESSE
      • The RCA
    • on Tools & Techniques
      • Big Data & Analytics
      • Experimental Design for NPD
      • Innovative Thinking in Reliability and Durability
      • Inside and Beyond HALT
      • Inside FMEA
      • Integral Concepts
      • Learning from Failures
      • Progress in Field Reliability?
      • R for Engineering
      • Reliability Engineering Using Python
      • Reliability Reflections
      • Testing 1 2 3
      • The Manufacturing Academy
  • eBooks
  • Resources
    • Accendo Authors
    • FMEA Resources
    • Feed Forward Publications
    • Openings
    • Books
    • Webinars
    • Journals
    • Higher Education
    • Podcasts
  • Courses
    • 14 Ways to Acquire Reliability Engineering Knowledge
    • Reliability Analysis Methods online course
    • Measurement System Assessment
    • SPC-Process Capability Course
    • Design of Experiments
    • Foundations of RCM online course
    • Quality during Design Journey
    • Reliability Engineering Statistics
    • Quality Engineering Statistics
    • An Introduction to Reliability Engineering
    • Reliability Engineering for Heavy Industry
    • An Introduction to Quality Engineering
    • Process Capability Analysis course
    • Root Cause Analysis and the 8D Corrective Action Process course
    • Return on Investment online course
    • CRE Preparation Online Course
    • Quondam Courses
  • Webinars
    • Upcoming Live Events
  • Calendar
    • Call for Papers Listing
    • Upcoming Webinars
    • Webinar Calendar
  • Login
    • Member Home

by Fred Schenkelberg 3 Comments

Special and Common Causes of Process Variation

Special and Common Causes of Process Variation

As stated before, variation happens.

The root cause of the variation for a stable process includes material, environmental, equipment, and so on, changes that occur during the process. No saw cuts the same length of material twice – look close enough there is some difference.

The difference between a stable and unstable process is the occurrence of special causes. A stable process has a predictable range of resulting dimensions and the variation is due to the inherent variability of the materials, machines, etc.

An unstable process experiences special causes. These are unique events which shift the process mean and/or increase the process standard deviation. Special causes may include using the wrong material, altering equipment settings or alignment, or even an increase in measurement error by using a damaged gauge.

Accidents, errors, mistakes, drift, wear, inattention, etc. all may lead to a change in the otherwise stable results of a process.

 

Special Causes

We notice some special causes even without control chart monitor. If we track scrap or failure rates we may notice spikes in the rate indicating something has gone wrong.

Special-Causes

The red circles indicate a signal that something is causing a deviation from normal. Often a team converges on the issue to identify and resolve the problem. This reactive approach missing one of the key benefits of control chart monitoring.

A control chart doesn’t eliminate the occurrence of special causes. It does shorten the time to detect the occurrence of special causes thus reducing scrap and the time necessary to resolve or remove the causes. Control charts are often located at one or more stations within a process thus closer to the likely source of the change. This enhances the ability to find the source of variation.

Special causes may also dramatically reduce scrap or failure rates. These events indicate an opportunity to improve the process.

Like adverse events, it takes investigation and understanding of the contributing factors so the process improvements remain in place and create a better and stable process.

Common Causes

A stable process may have a high scrap rate. And the process may not reveal beneficial special causes to prompt scrap rate reductions. In this case, the control chart may not provide what changes will cause improvements.

Common-Causes

Common causes of variation create the predictable range of readings seen from a stable process. A team focused on improving a stable process may need to investigate a wide range of possible causes before recommending ways to reduce variation or shift the mean to minimize scrap.

Tools such as a cause-and-effect diagram, fishbone diagram, or brainstorming will help identify potential causes. Then it may be simple experimentation, hypothesis testing, to determine if a change is effective or not. For more complicated combinations of causes and changes, the team may employ a design of experiments approach.

The changes may require different materials, equipment, procedures, or other systemic changes to realize improvements.

Dr. Deming suggests that 94% of all problems are due to common causes of variation. By focusing on reducing variation from the range of possible sources the process will produce fewer failures.

Making fundamental changes in design, materials, equipment and so on may require significant resources to accomplish, thus requires management support.


Related:

Sources of Variation (article)

Nelson Funnel Experiment (article)

Process Capability (article)

 

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Statistical Process control (SPC) and process capability

About Fred Schenkelberg

I am the reliability expert at FMS Reliability, a reliability engineering and management consulting firm I founded in 2004. I left Hewlett Packard (HP)’s Reliability Team, where I helped create a culture of reliability across the corporation, to assist other organizations.

« How to Select Tasks for a Reliability Plan
Set a reliability goal without MTBF »

Comments

  1. tony says

    February 8, 2015 at 9:36 PM

    a good point!

    Reply
  2. Ria says

    February 18, 2015 at 8:09 AM

    Hello Everyone,
    Can anyone please direct me to question banks for the CRE. I have been preparing for few months and want to evaluate myself now. If you have a simulator or question bank, I would highly appreciate if you could share it with me.
    Thanks

    Reply
    • Fred Schenkelberg says

      February 18, 2015 at 10:25 AM

      Hi Ria,

      Try asq.org and search for CRE – they have a 75 sample exam that you can take online or download and take. From what I understand it’s a bit tougher than the actual exam. Your mileage may vary.

      Cheers,

      Fred

      Reply

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

CRE Preparation Notes

Article by Fred Schenkelberg

Join Accendo

Join our members-only community for full access to exclusive eBooks, webinars, training, and more.

It’s free and only takes a minute.

Get Full Site Access

Not ready to join?
Stay current on new articles, podcasts, webinars, courses and more added to the Accendo Reliability website each week.
No membership required to subscribe.

[popup type="" link_text="Get Weekly Email Updates" link_class="button" ]

[/popup]

  • CRE Preparation Notes
  • CRE Prep
  • Reliability Management
  • Probability and Statistics for Reliability
  • Reliability in Design and Development
  • Reliability Modeling and Predictions
  • Reliability Testing
  • Maintainability and Availability
  • Data Collection and Use

© 2025 FMS Reliability · Privacy Policy · Terms of Service · Cookies Policy