Issue: Volume 9, Number 9
Date: September 2009
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, see below.

==> Tip: Get immediate answers to questions about DOE via the Search feature on the main menu of the Stat-Ease® web site. This not only pores over previous alerts, but also the wealth of technical publications posted throughout the site.

Feel free to forward this newsletter to your colleagues. They can subscribe by going to http://www.statease.com/doealertreg.html. If this newsletter prompts you to ask your own questions about DOE, please address them via mail to: [email protected].

For an assortment of appetizers to get this Alert off to a good start, follow this link, http://www.StatsMadeEasy.net* (-> new web site!), and see a number of new blogs (listed below, beginning with the most recent one):

—Making random decisions on the basis of a coin flip
—Are you happy? If so, be careful not to laugh: It may trigger gelotophobia!
—Regions with aging populations are experiencing higher death rates!

*Need a feed from StatsMadeEasy to Microsoft's Outlook? See http://office.microsoft.com/en-us/outlook/HA101595391033.aspx.

Also, Stat-Ease offers an interactive web site—its Support Forum for Experiment Design at http://forum.statease.com. Whereas this monthly ezine—the DOE FAQ Alert—shares one-on-one communications with Stat-Ease StatHelp, anyone (after gaining approval for registration) can post questions and answers to the Forum, which is open for all to see (with moderation). Check it out and weigh in!

The most recent materials posted are two announcements from Stat-Ease Programmer Tryg Helseth to the SOFTWARE SUPPORT "Network Issues (LAN versions)" section, which provide details on installing Design-Expert® software on a network and achieving annual license renewals.

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

1. FAQ: How do you know when to transform a response?
2. FAQ: Power issues with ruggedness testing
3. Info Alert: Published applications of DOE ranging from food science to corrosion inhibition
4. Book Giveaway: Winners announced
5. Webinar Alert (3rd): Problems Analyzing Historical Data
6. Events Alert: Introduction to Experimental Design & Automated Assay Optimization (AAO) FX Software
7. Workshop Alert: See when and where to learn about DOE
8. A heads-up on web-based statistics training: Statistics.com

P.S. Quote for the month: Senior economist takes a shot (?) at statisticians while commenting about our current recession.

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1. FAQ: How do you know when to transform a response?

-----Original Question-----
From:A German engineer
"Your help has been very useful. I was just wondering if you could tell me if there was a certain way you went about analyzing response surface method (RSM) data with Design-Expert or if you just have so much experience with it that you knew which transformations to do."

Answer (from Stat-Ease Consultant Shari Kraber):
"Some years ago, transformations used to be done by trial and error. Now the Box-Cox diagnostic plot* does the math for us and can quickly direct us to the best transformation. Here's what I generally do:

1. Choose the highest-order suggested model (sometimes two are advised by Design-Expert) and change the selection to apply a backwards model reduction.

2. Verify the model adequacy by looking over the analysis of variance (ANOVA) p-values and the adjusted and predicted R-squared values.

3. Check all diagnostics for abnormalities — especially the Box-Cox plot, which may advise a specific transformation.

4. If indicated, go to the Transform menu and apply the one recommended by Box-Cox. Then follow steps 1-3 again. Assess the adjusted and predicted R-squares — they should get at least a little higher with a good response transformation."

PS. Amazingly enough, in some cases you can get a better fit with a simpler model after a successful transformation. For example, see from Table 10-6* of "RSM Simplified" by Anderson & Whitcomb how applying a log to response data from a design and analysis of computer experiment (DACE) on the wing design for a lightweight fighter jet.

(Learn more about RSM and transformations by attending the computer-intensive workshop "Response Surface Methods for Process Optimization." See http://www.statease.com/clas_rsm.html for a description of this class and link from this page to the course outline and schedule. Then, if you like, enroll online.)

*Chapter 10 of this book is posted as a free sampler at http://www.statease.com/pubs/rsmsimpexcerpts--chap10.pdf.

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PLEASE DO NOT SEND ME REQUESTS TO SUBSCRIBE OR UNSUBSCRIBE—FOLLOW THE INSTRUCTIONS AT THE END OF THIS MESSAGE.

Sincerely,

Mark

Mark J. Anderson, PE, CQE
Principal, Stat-Ease, Inc. (http://www.statease.com)
2021 East Hennepin Avenue, Suite 480
Minneapolis, Minnesota 55413 USA

PS. Quote for the month —Senior economist takes a shot (?) at statisticians while commenting about our current recession:

"It's going to be a recovery only a statistician can love."

—Mark Vitner, Wells Fargo senior economist.

Trademarks: Stat-Ease, Design-Ease, Design-Expert and Statistics Made Easy 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 http://www.statease.com/consult.html for resumes)
—Statistical advisor to Stat-Ease: Dr. Gary Oehlert (http://www.statease.com/garyoehl.html)
—Stat-Ease programmers, led by Neal Vaughn and Tryg Helseth (http://www.statease.com/pgmstaff.html)
—Heidi Hansel Wolfe, Stat-Ease sales and marketing director, and all the remaining staff that provide such supreme support!

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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, #7-4 Apr 07, #7-5 May 07, #7-6 Jun 07, #7-7 Jul 07, #7-8 Aug 07, #7-9 Sep 07, #7-10 Oct 07, #7-11 Nov 07, #7-12 Dec 07, #8-1 Jan 08, #8-2 Feb 08, #8-3 Mar 08, #8-4 Apr 08, #8-5 May 08, #8-6 June 08, #8-7 July 08, #8-8 Aug 08, #8-9 Sep 08, #8-10 Oct 08, #8-11 Nov 08, #8-12 Dec 08, #9-01 Jan 09, #9-02 Feb 09, #9-03 Mar 09, #9-04 Apr 09, #9-05 May 09, #9-06 June 09, #9-07 July 09, #9-08 Aug 09, #9-09 Sep 09 (see above)

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