Issue: Volume 6, Number 9
Date: September 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

For an assortment of appetizers to get this Alert off to a good start, see the five new entries since last month's issue that I added to These include morbid data reported under the heading of "Stats that will be the death of you." I was reminded about these odds of dying from particular, many of them peculiar, causes upon hearing the sad news of crocodile hunter Steve Irwin's recent demise. Perhaps it may not be such a surprise for such a risk-taker, but what a way to go! :(

Topics in the body text of this DOE FAQ Alert are headlined below (ones that delve into statistical detail are designated "Expert"):

1. Faq: Why do you favor half vs full normal plot of effects?
2. Faq: How does the Pareto chart help choose effects?
3. Info Alert: Attention coatings experimenters — drawing for free handbooks that include a section on DOE (link to content)
4. Events alert: Many Stat-Ease appearances lined up this Fall
5. Workshop Alert: Fall schedule offers full DOE curriculum including an Experiment Design Made Easy workshop in Philadelphia

PS. Quote for the month: Who says these shocking words: "...anyone who used random numbers to guide his life should be hunted down and exterminated"? (Answer at the end of this e-mail!)


1. FAQ: Why do you favor half vs full normal plot of effects?

-----Original Question-----
From: Industrial statistician at a US-based medical device company "It appears from the defaults of your Design-Ease® and Design-Expert® software that you prefer half-normal plots of effects over their full-normal equivalent. The same is true in the NIST/SEMATECH e-Handbook of Statistical Methods — they detail the half-normal plot for choosing effects at Do you have any further comments on this issue?"

In the original versions of Design-Ease and Design-Expert dating back to the 1980's, Stat-Ease defaulted to the full normal plot. However, we gravitated to the half-normal plot, which displays the absolute value of effects, for two reasons:
— Users found it easier to identify the trivial many effects because they emanate from the origin
— It provides more sensitivity for seeing small, but possibly real, effects that fall off the line of near-zero effects.

I mentioned how much better half-normals are for picking effects at an ASQ talk attended by Stu Hunter, co-author of the landmark book, "Statistics for Experimenters." Afterwards he told me that Stat-Ease was right and that they (he and co-authors George Box and Bill Hunter) had it that way (half-normal) in their original manuscript, but just before publication the inventor of this graphical method for choosing effects, Cuthbert Daniel, wavered back to full-normal. Stu said their book went to press just after Daniel reversed himself back to favoring the half-normal again! Obviously, the choice of half versus full normal plots is debatable. That's why we provide the option for both in our software. The newest version* provides color-coding to see at a glance whether effects are positive or negative.

Do any of you readers have a strong preference?

*See for details and link from there to free 45-day fully-functional trials of version 7 of Design-Ease and/or Design-Expert software.


2. FAQ: How does the Pareto chart help choose effects?

(These questions were originally published in the February 2003 DOE FAQ Alert — )

From: User in United Kingdom "Mark, I am really interested in being able to get an ordered bar [Pareto] chart out of your software, which shows the importance of the effects in rank order as an alternative to the half-normal plot. I feel that this provides an intuitive way of identifying the most important effects. We have found the half- normal plot difficult to explain to scientists. Is there any way of doing this in Stat-Ease software, or any plans to put it into future versions?"

From: North Carolina statistician "Mark, with respect to future upgrades, may I suggest that a
Pareto chart of effects would be a very useful addition to the output. None of my associates uses a half-normal plot for a management group that generally has had no DOE training. They have to export to Excel, clean it up, make a Pareto chart and import again to the report document. While the utility of the half-normal plot is obvious to the analyst, it is a mystery to the unlearned and a distraction to explain. A simple Pareto chart would be a great time-saver and strengthen the output."

Ask and you shall receive! We not only added a Pareto chart to our DOE software, but made it into a tool for selecting effects by converting the response scale to t-values. See it pictured at However, Stat-Ease consultant Pat Whitcomb, the prime developer of this new tool, advises: "The problem with the Pareto chart is that you need to pick the obvious effects for the t-limits to be meaningful. The initial error estimate is inflated by factor effects. Proper t-limits are obtained only after the factor effects are removed from those pooled to estimate error. Therefore I suggest using it in conjunction with the half-normal plot. The half-normal plot is good for picking the obvious effects, but it is easy (particularly for a novice) to over-select effects. Switching to the Pareto chart after picking the obvious effects using the half-normal plot gives some guidance and helps prevent over selection. I like using the Pareto Chart to select the 'shoulder' effects; those that are neither obvious factor effects nor error terms."

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


3. Info alert: Attention coatings experimenters — Drawing for free handbooks that include a section on DOE (link to content)

(Sorry, due to the high cost of shipping, this offer applies only to residents of the United States and Canada.) Simply reply to this e-mail before September 12 if you'd like a chance at one of two free copies of the "Coatings Technology Handbook," 3rd Ed. (CRC). Winners, who will be drawn at random, will be notified and asked to supply their shipping address. This latest edition of the Handbook, which can be seen at , includes a new chapter (15) on "Design of Experiments for Coatings" authored by Pat Whitcomb and myself. You can see the content in our original manuscript posted at


4. Events alert: Many Stat-Ease appearances lined up this Fall!

Look over this line-up and see if any of our events scheduled this Fall will be convenient for you to attend and/or affiliated with a conference in your field of expertise:

- Minnesota Quality Conference, Bloomington, October 9-10, exhibit and pre-conference workshop "DOE/RSM Simplified" by me (for details, see

- National Association of Printing Ink Manufacturers Technical Conference, Ft. Lauderdale, FL, Oct 11, mixture design workshop by Pat Whitcomb (see

- 50th Fall Technical Conference — Statistics & Quality: 50 Years of Exploration & Discovery, Columbus, OH, October 12-13, exhibit and talk on "A Factorial Design Planning Process" by Shari Kraber and Pat Whitcomb (see and link from there to the program)

- MD&M Minneapolis, October 25-26, booth 954, conference site, click on to register for free exhibit hall admission

- Managing Improvement via Modern Design of Experiments, Stat-Ease workshop, Charlotte, NC, November 14, $349.00 (0.8 CEU's), print form at to register, fax when completed to 612.378.2152

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


5. Workshop alert: Fall schedule offers full DOE curriculum including an Experiment Design Made Easy workshop in Philadelphia

Here's the rundown of Stat-Ease public workshops this Fall season:

- Experiment Design Made Easy ("EDME"), September 26-28, Philadelphia, and again in Minneapolis on October 17-19

- Response Surface Methods for Process Optimization ("RSM"), September 26-28, Minneapolis

- Statistics for Technical Professionals ("STP"), October 4-5, Minneapolis

- Mixture Design for Optimal Formulations ("MIX"), October 24-26, Minneapolis

- Two-Day Crash Course on DOE for Sales & Marketing ("SMDOE2"), November 8-9, Minneapolis

If you are uncertain about which workshop would be best, see for the curriculum in hierarchical form. Click any workshop box for course details. Link from there to online enrollment in the next public class.

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 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. Source of shocking quote for the month — the antagonist, (a leader of a secret organization bent on establishing a perfectly controlled populace) in the paranoid-thriller novel "The Traveler" by John Twelve Hawks.
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 (see above)

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