Issue: Volume 3, Number 7
Date: July 2003
From: Mark J. Anderson, Stat-Ease, Inc. (

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 previous DOE FAQ Alerts, please click on the links at the bottom of this page. Feel free to forward this newsletter to your colleagues. They can subscribe by going to If this newsletter prompts you ask to your own questions about DOE, please address them to

Here are some good-old summertime appetizers to get this Alert off to a good start:

1. Click to see an unusual way to make ice cream suggested by Theodore Gray, a co-founder of Wolfram Research, who publishes Mathematica software.
2. If you decide to buy a cone instead and end up juggling change while trying to keep the ice cream from melting, consider how different denominations could save on coins:
—18 cents for USA
—83 cents for Canada
—1.33 or 1.37 Euros for Europe
Believe it or not, these would be optimal according to an article at
3. Flash! See (page no longer available) for startling news on lightning jets!

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: Graphing two responses to view correlation
2. Expert-FAQ: Why do replicated center-points in a central composite design (CCD) for response surface methods (RSM)?
3. Heads-up: Scary e-mails with a scientific flavor -- Read all about a typical one on the perils of plastic, but do not pass it on!
4. Events alert: Stat-Ease is exhibiting and hosting a roundtable discussion at the Joint Statistical Meetings in San Francisco
5. Workshop alert: We are coming to San Jose later next month, Philadelphia in September, plus much more

PS. Quote for the month—a more concise view from a Mars rocket scientist on "known unknowns", etc. (followup to last month's quote from Rumsfeld)


1. FAQ: Graphing two responses to view correlation

-----Original Question-----
From: Singapore

"Recently I bought Design-Expert® software. It is working well and caters to my needs. Presently, the software allows me to study interactions between the different factors for a specific response. I wonder if there is any way to view relationships between the different responses."

Answer (from Stat-Ease consultant Pat Whitcomb):
"When you are on the Design layout (the spreadsheet-like view) in Design-Expert (or Design-Ease® software) go to the main menu and choose View, Graph Columns. You can plot any two columns from your experimental matrix, including a pair of responses, against each other. Look for correlations."

P.S. The software reports the statistic "r"—a measure of correlation that varies from -1 (inverse relationship) to +1 (directly related). When "r" is zero, the two columns plotted are said to be "uncorrelated." This occurs when you perform standard two-level designs, or those laid out "orthogonally" via Taguchi.


(Learn more about basic statistics and two-level designs by attending the 3-day computer-intensive workshop "Experiment Design Made Easy." See for a complete description. Link from this page to the course outline and schedule. Then, if you like, enroll online.)


2. FAQ: Why do replicated center-points in a central composite design (CCD) for response surface methods (optimization)?

-----Original Question-----
From: Alabama

"When I set up a central composite design (CCD) for response surface methods (RSM), the software requested I run the center-point many times. Why should I do that? Would it be OK just to re-test the product, or do I have to re-make it?"

The CCD you requested from Design-Expert calls for 6 center-points intended to be run at random intervals with fabrication done from start to finish, not just re-sampled and/or re-tested. The book that Pat and I co-authored, "DOE Simplified" (see, provides an example of CCD (in Chapter 8) on confetti that I cut to varying dimensions before measuring flight time. In this case, I re-cut the confetti for each replicate of the center-points, the number of which is specified by statisticians to give a good balance of information in the middle of the space versus the outer regions. In a related study, Box and Liu ("Product Design with Response Surface Methods," Center for Quality and Productivity Improvement, Report No. 150, May 1998 at, cut paper helicopters but then re-dropped them many times. These repeated measures (not true replication of a run) then were averaged and this value entered as a response. Furthermore, Box and Liu computed the standard deviation and entered this as a second response to determine if some configurations proved to be more variable than others—an issue related to stable vs. unstable aircraft design.

For more details, see a write-up by fellow Stat-Ease consultant Pat Whitcomb in the June 1998 Stat-Teaser entitled "Center-Points in Central Composite Design" at

P.S. Another Stat-Ease consultant, Shari Kraber, commented:
"Replication of center-points provides these advantages:
—Reduces the prediction error in the presumed optimum of the design space. (It's assumed that the CCD will be centered on the 'optimum', so you want the best prediction capability there.)
—Provides pure error, used for the lack-of-fit (LOF) test. Simply re-testing the product underestimates the pure error, making the LOF test inaccurate and therefore worthless."

(Learn more about RSM designs by attending the "Response Surface Methods for Process Optimization" workshop. For a description, see Link from this page to the course outline and schedule. You can enroll online by linking to the Stat-Ease e-commerce page for workshops.)


3. Heads-up: Scary e-mails with scientific flavor -- Read all about typical one on perils of plastic, but do not pass it on!

-----Original Question-----
From: Non-scientific person

"I just received an e-mail that worries me very much. It says that plastic wrap causes cancer. Can you find out if this is true?"

Answer (from Stat-Ease consultant Pat Whitcomb):
My colleague Tryg helped me track down a web site that reproduces the widely-circulated letter with the supposed studies showing carcinogenic properties of plastic wrap and the rebuttal. See it for yourself at Go back to the main site to clarify the truth (if any) behind other Internet-spread rumors which seem to run rampant all over the globe.

Here's my first response to the question, before Tryg found the specific answer:
"It would take a lot of digging to follow-up on all the issues brought up by this inflammatory e-mail. I'd be very surprised if there's anything to it. Generally these things start with enough elements of truth to sound plausible but from there they play on fears in an unscrupulous way.

It boils down to this: Do not believe anything you read that's not reviewed by peers, such as technical articles in reputable scientific journals. Even reputable scientists may publish results that later prove to be erroneous, so (in my opinion!) it pays to be skeptical at all times about everything."

I received this reply from the inquirer:
"Thank you! I didn't forward it along and I'm glad now."

In my opinion, anything you people reading this newsletter can do to quell rumors like this would be a service to humankind. Mark


4. Events alert: Stat-Ease exhibiting and hosting roundtable discussion at Joint Statistical Meetings in San Francisco

Second notice:
See for details on this year's Joint Statistical Meetings (JSM) on August 3-7, 2003 in San Francisco, California. JSM is the largest gathering of statisticians held in North America. It is sponsored by the American Statistical Association (ASA), the International Biometric Society (ENAR and WNAR), the Institute of Mathematical Statistics, and the Statistical Society of Canada. Activities include "oral presentations, panel sessions, poster presentations, continuing education courses and an exhibit hall with state-of-the-art statistical products" (such as Design-Expert software at booth 205!). A Stat-Ease consultant will lead roundtable Session 140 over lunch on Monday 12:30 PM, sponsored by the Section on Quality & Productivity. The topic is "Practical versus Statistical Aspects of Altering Central Composite Designs." See for more details. It will be interesting to share knowledge with expert practitioners of response surface methods (RSM).

Click for a list of where Stat-Ease consultants will be giving talks and doing DOE demos. We hope to see you sometime in the near future!


5. Workshop alert: Coming to San Jose later next month, Philadelphia class in September, plus much more

Second notice:
I am slated to teach the August 5-7 Experiment Design Made Easy (EDME) workshop in San Jose, California. I hope to see a good turn-out of students from the Far West and elsewhere. Mark

For you East Coasters we offer Experiment Design Made Easy on September 9-11 in Philadelphia, PA. For other workshops and other locations (mainly at our home training site in Minneapolis), see for a schedule. 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. (
Minneapolis, Minnesota USA

PS. Quote for the month—a more concise view from a Mars rocket scientist on "known unknowns", etc. (followup to last month's quote from Rumsfeld):

"We want to research what we call the 'known unknowns.' This will reduce total risk in the face of unknown unknowns, the true surprises out there."

—John Charles, senior staff scientist at NASA's Space and Life Sciences Directorate, speaking about a possible voyage by humans to Mars

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 and Shari Kraber (see for resumes)
—Statistical advisor to Stat-Ease: Dr. Gary Oehlert (
—Stat-Ease programmers, especially Tryg Helseth (
—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 (see above)

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