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 http://www.statease.com/doealertreg.html. If this newsletter prompts you ask to your own questions about DOE, please address them to firstname.lastname@example.org.
Here's an appetizer to get this Alert off
to a good start:
1Info alert: The June issue of the Stat-Teaser (link provided) features a DOE semifolded to minimize misses from medieval missile machines
Many of you by now have received a printed copy of the latest Stat-Teaser, but others, by choice or because you reside outside of North America, will get your first look at the June issue at http://www.statease.com/news/news0306.pdf. (Adobe recently upgraded their Reader software to version 6. For a free download, click http://www.adobe.com/products/acrobat/readstep2.html. You need Reader for viewing files in "pdf"Portable Document Format.)
The feature article, "Messing with Medieval Missile Machines" describes how I uncovered the secrets to operating a simulated trebuchet operated by MoreSteam.com, a provider of web-based Six Sigma training. As a special favor to Stat-Ease, the developer of the trebuchet DOE-simulator, Bill Hathaway, offers temporary access at http://www.moresteam.com/trebuchet_se.cfm. For more details on the simulator, see item #3 in the January 2002 Alert posted at http://www.statease.com/news/faqalert2-1.html. At that time, the trebuchet was still at the beta-testing stage, so it was made available to readers of the DOE FAQ Alert.
The other story in the Stat-Teaser, authored
by consultant Shari Kraber, provides details on the DOE that I used
on the trebuchet. Her article is entitled "SemifoldingMore
Information, Fewer Runs."
2FAQ: A half-fraction on 3 factors at two-levels does not produce much information
However, Log transformation was recommended. After applying a Log transformation, the model became significant and so did the factors B and C.
As I had picked factors B and C because they are significant, why was the Model Graph showing only the One Factor Plot and not the Interaction Graph? Is it because there is no real interaction between factors B and C? Even if it is so, I would still find it useful if the Interaction Graph can be made a default. Is there any way of making it so?"
You went ahead and chose B(=AC) and C(=AB) for your model. ANOVA shows this to be significant relative to the remaining estimable effect of A(=BC). However, with such meager data, I would not put much faith in the statistics! The software then only shows one-factor plots of B and C, because that is what you put in the predictive model. To get the interaction of BC you must go back to Effects and click A into the model (you can also do this via View, Effects List). Then go to View, Alias List, right-click on A and change it to its alias BC. Then do ANOVA (no term now for residual, so ignore this) and press ahead to Model Graphs.
I advise that you not choose this design again. Instead perform the full 2^3 factorial. Furthermore, if you are studying numerical factors that may be near optimum levels, consider adding 4 center-points so you can test for curvature. These additional runs will also provide an estimate of pure error, thus strengthening your DOE considerably.
(Learn more about fractional two-level designs by attending the 3-day computer-intensive workshop "Experiment Design Made Easy." See http://www.statease.com/clasedme.html for a complete description. Link from this page to the course outline and schedule. Then, if you like, enroll online.)
3FAQ: Treating multilevel factors as categorical versus numerical
Answer (from Stat-Ease
consultant Pat Whitcomb):
I'd just like to add that users of Design-Ease® or Design-Expert software are allowed to change categorical factors by right-clicking over the header and choosing Make Numeric. (Mark)
4Info alert: The May issue of Quality Progress features a DOE that uncovered the cause for bread not rising in a kitchen machine
The May issue of Quality Progress features:
Ruggedness Testing To Prevent Failures
See this and other abstracts for articles
that month at http://www.asq.org/pub/qualityprogress/past/0503/.
However, to view the magazine online you must be a member of the American
Society of Quality (ASQ). Otherwise see the original manuscript at
5Events alert: Stat-Ease is exhibiting and hosting a roundtable discussion at the Joint Statistical Meetings in San Francisco
See http://www.amstat.org/meetings/jsm/2003/ 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 http://makeashorterlink.com/?Z336269C4 for more details. It will be interesting to share knowledge with expert practitioners of response surface methods (RSM).
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!
6Workshop alert: Coming to San Jose later this Summer
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. We've
got lots of other classes coming up, mostly in Minneapolis, but in
other cities too. See http://www.statease.com/clas_pub.html
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.
7Letter from reader: Inspiring comments about success with DOE
From: Philip Kneisl,
Senior Chemical Engineer, Texas
I hope you learned something from this issue. Address your general questions and comments to me at:
Mark J. Anderson, PE, CQE
PS. Quote for the
monthWise words on screening for the unknown from a US
government official (I challenge you to make sense of this press statement):
Trademarks: Design-Ease, Design-Expert and Stat-Ease are registered trademarks of Stat-Ease, Inc.
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