Issue: Volume 6, Number 4
Date: April 2006
From: Mark J. Anderson, Stat-Ease, Inc. (
"Statistics Made Easy" (tm) 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

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Here's an appetizer to get this Alert off to a good start: — an in-class experiment that compares the flavor longevity of two different brands of chewing gum. Gum manufacturers seek the longest-lasting flavor — about 12 minutes is typical according to statistics cited by the authors. (After that time is up it goes under the classroom desk, so ideally the flavor could be stretched longer than the lecture and students would keep their gummy where their mouths are.)

I found a more sophisticated experiment on how long gum lasts at
It suggests a two-level factorial design on:
A. Flavor — Fruit Juice versus Double Mint
B. Gender — Female vs Male
C. Meal — Before vs After.
The author, Gerardine G. Botte (currently at Ohio University, suggested students at the University of Minnesota Duluth chew on this as homework for their Design of Engineering Experiments class. Many other fun ideas for DOE exercises are provided by Professor Botte. I wish I had her as a statistics teacher!

PS. Breaking news: Chewing gum makes you smarter and good-looking! Wrigley gum researchers hope to prove this by investing in unbiased (?) experimentation on these possible benefits to masticating their product. See for their publicity on this research initiative.

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: Why is there no intercept in models for mixture designs?
2. FAQ: Categorical factor in a response surface method (RSM)
3. Info Alert: Balloon car simulation (submitted by reader)
4. Submission: Photo of "RSM Simplified" book at Center Point
5. Events Alert: First European DOE User Meeting (2nd notice!)
6. Workshop Alert: "Crash Course on DOE for Sales & Marketing"

PS. Quote for the month: How to get the Net Generation engaged in statistics classes.


1. FAQ: Why is there no intercept in models from mixture designs?

-----Original Question-----
From: Pennsylvania
"Why do I not have the intercept as in a regular factorial model in my analysis of a mixture experiment set up on Design-Expert® software?"

We make use of a mixture model developed by Scheffe, which is characterized by not having an intercept or squared terms for the quadratic form. For the mathematical details, see these lecture notes by Professor Steve Buyske (link provided with his express permission) from the Statistics Department of Rutgers University: As he states "...Design-Expert will analyze mixture designs with aplomb..."

(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 a schedule of public presentations and on-line enrollment.)


2. FAQ: Categorical factor in a response surface method (RSM)

-----Original Question-----
From: Ohio
"I saw your DOE FAQ Alert electronic newsletter on the internet and I was wondering if I could ask you a question about an issue I am having with setting up a DOE design. I am trying to set up a four-factor, three-level design. Since it is three-level, (I know that the response is nonlinear), I wanted to use a central composite design (CCD), but one of my factors, "types of acids", is qualitative. I have three different types of acids that I want to try. The other three factors are quantitative. I realize that the CCD design is only for quantitative factors, but since I have three different acids types can I still use the CCD design?"

Yes, but it must be a three-factor CCD replicated on each of the three acids, or, more efficiently, a D optimal fraction aimed at fitting the four-factor quadratic model — the backbone of response surface methods (RSM). Our software Design-Expert Version 7 provides the necessary tools (free trial available at

(Learn more about RSM by attending the three-day computer-intensive workshop "Response Surface Methods for Process Optimization." See for a complete description. Link from this page to the course outline and schedule. Then, if you like, enroll online.)


3. Info alert: Balloon car simulation (submitted by reader)

-----Original Submission (edited)-----
From: Anish Patani, GE Energy, Master Black Belt - Lean & Six Sigma Program Manager
"I haven't had an opportunity to use Design-Expert but the reviewer you cited last month is right on by saying that "Stat-Ease is known for the jokily accessible examples." (See Any form of humor to make statistics fun is always welcome. Reading your post "DOE It Yourself, Fun Science Projects" at on in-class DOEs, I got the drag racing software, but it took too much time for students to learn. A student in my class suggested the Balloon Car Simulation ( that they had used in a MBA class. Each student was required to run all possible combinations (60) to get the optimum answer. The simulation is very simple — you can vary four factors:
— Tire size (2)
— Axle length (2)
— Valve diameter (5)
— Side wall height (3)

The objective is to increase speed and maximize distance. By running a half-factorial DOE you can get two transfer functions for speed and distance which yields an optimum or close to optimum answer. Then one can perform validation runs for the remaining eight runs in addition to creating a full-factorial design.

— If you ask the student to find the best speed and distance without teaching DOE, the chances are they will reach the optimum in 15 trials but they will continue trying further experiments without realizing they have reached the optimum design.
— The two transfer functions contradict each other and now the solution becomes a question about optimization: What will you compromise — speed or distance?
— No interactions are significant, hence using one factor at a time or sticking with the winner approach works in this case, but if there were interactions the student needs to understand DOE.
— For the factor valve diameter the response is not linear, hence if you selected two extreme settings, the chances are you will not get optimum transfer function, but if you selected the 2nd and 4th levels it will be better, yet not the best. This should take you into a discussion on selection of regions such that the assumption of linearity is valid, or consideration for a central composited design (CCD) for generating a non-linear response surface.
Caution — since the simulation is very simple so is the learning. It is good simulation for those who are doing DOE for the first time. For more advanced learning, one can use the simulation for drag racing that you suggest in "DOE It Yourself."

I took a look at this — it promises to be fun! I plan to actually make some balloon cars to see what they will do.


4. Submission: Photo of "RSM Simplified" book at Center Point

-----Original Submission (edited)-----
From: Ian A. McCulloh, Major, Chemical Corps, Instructor
"Mark, I teach RSM at the U.S. Military Academy at West Point.I bought your "RSM Simplified" book* for my students this semester. A tradition here at the Academy is for students to get a spring break picture with their text book. The idea is to try and make them at least take a text book with them. There is some obvious RSM/DOE humor, but I thought you might like to put it in the Stat-Ease newsletter."

Click the following link to see how CDT Dusty Turner carried on the U.S. Military Academy tradition for spring break: According to Major McCulloh "he is a second semester junior or COW as we call it here. He was born and raised in Center Point, TX." I found 18 "Center Point"s listed for the USA in Microsoft "Streets and Trips" . I suppose that wherever one is must be considered the center point of that person's universe.

*(See for details on this book and link from there to order online.)


5. Events alert: First European DOE User Meeting (2nd notice!)

The "First European DOE User Meeting" for Design-Expert and Design-Ease Users" will be held at the Faculty Club in Leuven, Belgium on April 24 and 25. This DOE conference is sponsored by Stat-Ease with hosting by CQ Consultancy and participation from other European resellers of Design-Expert and Design-Ease software. See details at The meeting is a must for European users of Stat-Ease software and perhaps the perfect excuse for those elsewhere to take in the sights in Leuven and surroundings. I lunched at the city center some years ago — it was very beautiful!

Also, coming up soon are:
— ASQ's "World Conference on Quality & Improvement" in Milwaukee, Wisconsin on May 1-3, where I will help staff the Stat-Ease exhibit at Booth 424.
— PSI (statisticians in the pharmaceutical industry) 29th Annual Conference 2006 in Bristol, UK on May 14-17, where reseller and DOE expert in his own right, Alan Collins, will exhibit Stat-Ease software via his company, QD Consulting.

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


6. Workshop alert: Crash Course on DOE for Sales & Marketing

(If you are a corporate technical professional, please pass this news on to your business people!) Quickly identify those factors which affect your sales and marketing results. Learn how they interact and apply that knowledge to make breakthrough increases in sales and profits. Attend the newly expanded two-day "Crash Course on DOE for Sales & Marketing" at the Stat-Ease training center in Minneapolis on April 19 20. See the course description and links to the syllabus and online enrollment at Note that we will also present a one-day version of DOE for Sales & Marketing at a hotel near O'Hare airport in Chicago this June.

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 to get the Net Generation (see engaged in statistics classes:

"Their fast-paced world of action movies, rapid-fire TV commercials and video games does not prepare today’s students to sit and absorb a lecture, especially on a supposedly dull subject like statistics. To capture the interest of these students, teaching must move away from lecture and listen to innovative activities that engage students in the learning process."

—R. Scheaffer

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
—Fellow 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


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#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, #6-04 Apr 06 (see above)

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