Issue: Volume 7, Number 3
Date: March 2007
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

For an assortment of appetizers to get this Alert off to a good start, see these new blogs at
— Do narrower columns hold up better for body of written work?
— Graupeling for words to describe nature's emanations
— Overreacting to patterns generated at random — Part 2
— Overreacting to patterns generated at random — Part 1
— Weather to be or not to be, that is the question

Topics in the body text of this DOE FAQ Alert are headlined below (the "Expert" ones, if any, delve into statistical details).

1. Announcement: New release of Design-Expert® software
2. Expert-FAQ: Dimples in my standard error plot
3. Events Alert: World Conference on Quality & Improvement
4. Workshop Alert: Across the USA from Philadelphia to San Jose

PS. Quote for the month: Common sense advice by DOE guru George Box on why one must experiment. (Page through to the end of this e-mail to enjoy the actual quote.)


1. Announcement: New release of Design-Expert® software

Stat-Ease announces a new release — version 7.1 of Design-Expert software (DX7.1). It beefs up power and design capabilities and offers clever new interface features that make design and analysis easier than ever. For a free, fully-functional trial, click and/or view tutorials at See pricing for new licenses or upgrades at our e-commerce site: Here are the highlights on what's new in DX7.1:

• Upfront power calculation for factorial designs: Avoid the embarrassment of running an experiment that has little chance of detecting an important effect.
• Fraction of design space (FDS) graph for design evaluation: Simple enough that even non-statisticians can see differences at a glance and applicable to any type of experiment — mixture, process or combined, this enhancement provides very helpful information for choosing from alternative test matrices.
"...very valuable in studying the potential performance of a design..."
Doug Montgomery (Editorial on "Designed Experiments in Process Improvement," Quality and Reliability Engineering International, 2006, 22: 863-864)

• Bookmarks for informative, but lengthy reports: Jump immediately to just the statistics you need to see.
• Grid lines on 3D-graph back-planes: Gain valuable perspective on optimal regions of your response surface.
• Save graphs to encapsulated postscript (EPS), PNG, JPEG and several formats: Facilitate publication of your exciting discoveries.
• Freely add columns for point type and other informative attributes to the design layout: Make your "recipe" sheet more helpful to the actual experimenters.

• Screening designs now available up to 50 factors: Discover significant main effects in the least runs possible while maintaining a balance in low versus high levels.
• Characterization design catalog for minimum-run ("Min Run") builder also expanded to 50 factors: Resolve two-factor interactions (2FI's) at an affordable cost.
• Central composite designs (CCD) increased to 50 factors: This widens the horizon for quickly generating useful transfer functions from costly computer simulations based on finite element analysis and the like.
• Simple ratio constraints, such as A/B>1, can be entered in directly: This sort of thing is fairly common, for example, A might be air pressure upstream of a check valve and B the pressure after, but it will work only when A exceeds B.

• Write transfer functions in format (.vta) readable by VarTran® software (Taylor Enterprises): Popular with those in design for six sigma (DFSS) Programs, this capability sets the stage for statistical tolerancing and sensitivity analysis.

For additional new features in v7.1 and a complete detailing of Design-Expert, view To get further explanation, call 1.612.378.9449 and ask for a Stat-Ease statistical consultant.


2. Expert FAQ: Dimples in my standard error plot

-----Original Question-----
From: India
"I have tried a central composite design with two factors at three levels. Why am I not getting a circular contour plot as shown in the February 2004 FAQ 1 on 'Interpreting the standard error plot'*? Instead I see four dimples at the corners."

Answer (from Stat-Ease Consultant Pat Whitcomb):
"The standard error (SE) plot is a function of the number of runs and where they are positioned. The defaults Design-Expert uses for a two factor CCD are:
— 1 each of the four factorial runs (2^2) at ±1 coded levels,
— 1 each of the four axial (star) runs at ±1.41,
— 5 replicates of the center point (0 in coded space).
This design is rotatable and thus it produces the circular SE plot noted in the FAQ you refer to. Your design, however, is different; It has the axial points pulled in to ±1 — thus making it a face-centered design (FCD) with three levels for each of your factors (as noted in our original question.) The reduced range in axial points increases the SE on the perimeter of the design space, particularly on the centers of the edges.**

To make matters even worse, your design has only one center point rather than the five that our software recommends. Decreasing the number of center points naturally increases the SE in the center of the design space. This generates a mound in the center that combined with the high SE on the perimeter creates
'dimples' on the contour plot."

** See the dimply contour plot and 3D view from this altered FCD at
— Mark

(Learn more about central composite design options by attending the three-day computer-intensive workshop "Response Surface Methods for Process Optimization." For a description of this class, see and link to the course outline and schedule. Then, if you like, enroll online.)


3. Events Alert: World Conference on Quality & Improvement

I will be attending the American Society of Quality (ASQ) World Conference on Quality & Improvement (WCQI) in Orlando, Florida, on April 30 through May 2 this Spring ( Stat-Ease Marketing Director Heidi Hansel will be joining me at Booth 331 in the WCQI exhibit hall. On Monday night that week, the Chemical and Process Industries Division (CPID) of ASQ offers an open meeting that will feature talks of a more technical nature than those generally given at WCQI. One will be given by me on the application of DOE to validation, for example, on quality control methods. It's titled "Design Of Experiments for Ruggedness Testing — Power Versus Resolution." For details on this CPID session for WCQI, contact me via Pat Whitcomb will give a talk at the Annual 2007 Quality &
Productivity Research Conference in Santa Fe, New Mexico, on June 4-6. See for details. Pat will also set up a table-top display. Click for a list of appearances by Stat-Ease professionals. We hope to see you sometime in the near future!

PS. Do you need a speaker on DOE for a learning session within your company or technical society at regional, national, or even international levels? If so, contact me. It may not cost you anything if Stat-Ease has a consultant close by, or if a web conference will be suitable. However, for presentations involving travel, we appreciate reimbursements for airfare, hotel and meals — expenses only. In any case, it never hurts to ask Stat-Ease for a speaker on this topic — we are at the foremost ranks of practical expertise on design of experiments for process and product improvement. Contact me at if you have an event coming up with an open slot for a presentation.


4. Workshop Alert: Across the USA from Philadelphia to San Jose

Stat-Ease will be seen from coast-to-coast this Spring via these two computer-intensive workshops:
— "Experiment Design Made Easy" in Philadelphia, PA, on March 27-29. See for course content and link from there to enroll online.
— "Mixture Design for Optimal Formulations" in San Jose, CA, on April 17-19. Details on this class and the signup link are provided at 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.*

*Once you achieve a critical mass of about 6 students, it becomes very economical to sponsor a private workshop, which is most convenient and effective for your staff. For a quote, call and ask for Workshop Coordinator, Sherry Klick.


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 monthCommon sense advice by DOE guru George Box on why one must experiment:

"To find out what happens when you change something, it is necessary to change something."
George Box
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, #6-8 Aug 06, #6-9 Sep 06, #6-10 Oct 06, #6-11 Nov 06, #6-12 Dec 06, #7-1 Jan 07, #7-2 Feb 07, #7-3 Mar 07 (see above)

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