Issue: Volume 7, Number 6
Date: July 2006
From: Mark J. Anderson, Stat-Ease, Inc., "Statistics Made Easy" ™ Blog

Dear Experimenter,

Here's another set of frequently asked questions (FAQs) about doing design of experiments (DOE), plus alerts to timely information and free software updates. If you missed the previous DOE FAQ Alert, please click on the links at the bottom of this page. If you have a question that needs answering, click the Search tab and enter the key words. This finds not only answers from previous Alerts, but also other documents posted to the Stat-Ease web site.

Feel free to forward this newsletter to your colleagues. They can subscribe by going to If this newsletter prompts you to ask your own questions about DOE, please address them via mail

Here's an appetizer to get this Alert off to a good start: See for an "Experi-Mento" video demonstrating an explosive reaction that requires only candy and cola. To view this you need Quicktime and a high-speed internet connection — give it a few moments to load. In any case, get to the source of this experiment at and consider trying it at your home, or better yet, outdoors!

Here's what I cover in the body text of this DOE FAQ Alert (topics that delve into statistical detail are designated "Expert"):

1. FAQ: Requirements for lack-of-fit test
2. Info Alert: A colorful application for mixture design
3. Reader Response: Comments on Cornell's ideas on how to assess the impact of blocking
4. Unsolicited Testimonial: "Most practical stat books ... ever"
5. Events Alert: Stat-Ease seen coast to coast this Summer
6. Workshop Alert: See when and where to learn about DOE

PS. Quote for the month: How the famous author Jane Austen generated food for her fascinating writing.


1. FAQ: Requirements for lack-of-fit test

-----Original Question-----
New Jersey
"Thanks to the Stat-Ease staff of consultants for your help using Design-Expert® software to set up a D-optimal factorial design. My statistically-savvy supervisor was quite impressed by the way your program suggests the addition of lack-of-fit points and replicates for an estimate of pure error. Can you explain this to me in a little more detail: What does this mean for the overall results?"

Answer (from Stat-Ease Consultant Shari Kraber): "Let me clarify. In order to do a lack-of-fit test, there are two things required:

1. The design must include some replicated points for estimating pure error.

2. "Extra" unique design points, beyond the minimum needed for the model, must be included. Design-Expert calls these points "lack of fit." It positions them as far away from other points as possible using a distance-based criterion.

A good predictive model will fit the D-optimally selected and the lack-of-fit points equally well. The lack-of-fit test compares the difference between the actual and predicted values to the pure error between the replicates. Ideally you will find any lack of fit to be statistically insignificant.

The recently-released version 7 of Design-Expert (DX7) features a 3D bar graph* that displays the observed response values above and below the fitted surface. We have found this to be a great way to visualize the model-fit.

*PS: See this graph on the last page of the DX7 tutorial posted at
— Mark

(Learn more about lack of fit and other DOE basics by attending the three-day computer-intensive workshop "Experiment Design Made Easy." See for a course description. Link from this page to the course outline and schedule. Then, if you like, enroll online.)


2. Info alert: A colorful application for mixture design

I should write "colourful" per the preference of Dr. Timothy Ebert. He is an entomologist and a practicing potter with an interest in how science can enhance art. This inspired Tim to apply mixture design for optimal formulation of colors for a ceramic glaze. His work was published under the title "Designing Glaze Colours" in issue 21 of Ceramics Technical journal (, pages 30-37. Tim's version of this publication can be seen at

(Learn more about design and analysis of mixture experiments by attending the three-day computer-intensive workshop "Mixture Design for Optimal Formulations." For a course description, click and link from there to our schedule of public presentations and online enrollment.)


3. Reader response: Comments on Cornell's ideas on how to assess the impact of blocking

-----Original Question-----
Philip Dixon, Professor of Statistics, Iowa State University
First off, congratulations for a wonderful newsletter. My applications are primarily biological and agricultural, but there are always interesting and useful comments.

I'm motivated to write by John Cornell's comments on blocking.* Like John, I emphasize that every experiment suggests how to improve future experiments. I also find it very useful to evaluate whether to continue using the same types of blocks. I don't use a formal test, however. There is a useful addendum to the Cochran and Cox expression for the efficiency of blocking. That is:

Eff(RCBD to CRD) > 1 if and only if F (=MSBlk/MSErr) > 1

The formal p-value is less useful than the estimated F statistic. In fact, insisting on a 'significant' block effect is too restrictive."

*See #4 at


4. Unsolicited testimonial: "Most practical stat books ... ever"

-----Original Comments-----
From: Design-Expert software user in Kansas
"I have purchased your two books "DOE Simplified" and "RSM Simplified." They are the most practical stat books I have ever bought — excellent work. These books should be recommended for any college course on process improvement. You guys just need "Mixture Simplified" and I will be in stat heaven!"

Answer: My co-author Pat Whitcomb and I are grateful for these complimentary comments and others of its kind. Many have asked that we move on to mixture design. We agree wholeheartedly that this would be helpful for those who do not need something as thorough as the texts by Cornell and Smith. However, our first task is to update "DOE Simplified" — a second edition of which is now well underway.

(For books on DOE, see Click on the picture of any book for detail on its content. Then purchase it directly via e-commerce.)


5. Events alert: Stat-Ease seen coast to coast this Summer

I will be on the USA East Coast on July 30 to explain how Stat-Ease software facilitates "Design and Optimization of Growth Medium." This is the title of a preconference workshop presented for the 2006 Annual Industrial Microbiology and Biotechnology Meeting in Baltimore, MD. For details, see #3 at

Stat-Ease will make an appearance on the West Coast at the Joint Statistical Meetings (JSM) of the American Statistical Association (ASA) and several other professional societies next month in Seattle, WA. Consultant Wayne Adams will chair a roundtable discussion on "Graphical & Numerical Approaches to Selecting Effects in Two-Level Factorial Models" ( ) over lunch on Tuesday August 8. It is sponsored by the Section on Quality and Productivity (Q&P). Stat-Ease will exhibit its software at Booth 318 on Sunday, August 6th through Wednesday the 9th.

Click for a list of appearances by Stat-Ease professionals. We hope to see you sometime in the near future!


6. Workshop alert: See when and where to learn about DOE

See for schedule and site information on all Stat-Ease workshops open to the public. To enroll, click the "register online" link on our web site or call Stat-Ease at 1.612.378.9449. If spots remain available, bring along several colleagues and take advantage of quantity discounts in tuition, or consider bringing in an expert from Stat-Ease to teach a private class at your site. Call us to get a quote.


I hope you learned something from this issue. Address your general questions and comments to me at:



Mark J. Anderson, PE, CQE
Principal, Stat-Ease, Inc. (
2021 East Hennepin Avenue, Suite 480
Minneapolis, Minnesota 55413 USA

PS. Quote for the month — How the famous author Jane Austen generated food for her fascinating writing:

"One cannot fix one's eyes on the commonest natural production without finding food for a rambling fancy."

— Jane Austen (I just returned from a vacation with my wife and three daughters in Bath, England where we visited the Jane Austen house ( While living there, Jane wrote "Pride and Prejudice" — familiar to many because of the popular and critically-acclaimed 2005 movie starring Keira Knightley. I recommend it! - Mark)

Trademarks: Design-Ease, Design-Expert and Stat-Ease are registered trademarks of Stat-Ease, Inc.

Acknowledgements to contributors:
—Students of Stat-Ease training and users of Stat-Ease software
—Stat-Ease consultants Pat Whitcomb, Shari Kraber and Wayne Adams (see for resumes)
—Statistical advisor to Stat-Ease: Dr. Gary Oehlert (
—Stat-Ease programmers, especially Tryg Helseth and Neal Vaughn (
—Heidi Hansel, Stat-Ease marketing director, and all the remaining staff


Interested in previous FAQ DOE Alert e-mail newsletters?
To view a past issue, choose it below.

#1 Mar 01, #2 Apr 01, #3 May 01, #4 Jun 01, #5 Jul 01 , #6 Aug 01, #7 Sep 01, #8 Oct 01, #9 Nov 01, #10 Dec 01, #2-1 Jan 02, #2-2 Feb 02, #2-3 Mar 02, #2-4 Apr 02, #2-5 May 02, #2-6 Jun 02, #2-7 Jul 02, #2-8 Aug 02, #2-9 Sep 02, #2-10 Oct 02, #2-11 Nov 02, #2-12 Dec 02, #3-1 Jan 03, #3-2 Feb 03, #3-3 Mar 03, #3-4 Apr 03, #3-5 May 03, #3-6 Jun 03, #3-7 Jul 03, #3-8 Aug 03, #3-9 Sep 03 #3-10 Oct 03, #3-11 Nov 03, #3-12 Dec 03, #4-1 Jan 04, #4-2 Feb 04, #4-3 Mar 04, #4-4 Apr 04, #4-5 May 04, #4-6 Jun 04, #4-7 Jul 04, #4-8 Aug 04, #4-9 Sep 04, #4-10 Oct 04, #4-11 Nov 04, #4-12 Dec 04, #5-1 Jan 05, #5-2 Feb 05, #5-3 Mar 05, #5-4 Apr 05, #5-5 May 05, #5-6 Jun 05, #5-7 Jul 05, #5-8 Aug 05, #5-9 Sep 05, #5-10 Oct 05, #5-11 Nov 05, #5-12 Dec 05, #6-01 Jan 06, #6-02 Feb 06, #6-03 Mar 06, #6-4 Apr 06, #6-5 May 06, #6-6 Jun 06, #6-7 Jul 06 (see above)

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