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. 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 http://www.statease.com/doealertreg.html. If this newsletter prompts you to ask your own questions about DOE, please address them via mail to: StatHelp@StatEase.com.
Here's an appetizer to get this Alert off to a good start—a thermodynamic toy called the Film Can Cannon:
http://www.scitoys.com/scitoys/scitoys/thermo/thermo2.html. I first saw it at my local library in a new book on science toys called "Gonzo Gizmos: Projects & Devices to Channel Your Inner Geek" by Simon Quellen Field (available at Amazon). He also maintains this web site on ingredients in stuff you find at home: http://sci-toys.com/ingredients/ingredients.html . Check it out—Quellen has done a lot of homework on components in foods and chemicals in household products.
Here's what I cover in the body text of this DOE FAQ Alert (topics that delve into statistical detail are designated "Expert"):
1. Newsletter Alert: The September issue of the Stat-Teaser features "Playing with Paper Helicopters"
2. FAQ: Diagnosing residuals from a DOE
3. Free books (US/Canada only): See the winners of the drawing for Box and Draper's "Empirical Model-Building and Response Surfaces"
4. Info alert: Wanted—case study authors
5. Events alert: Link to a schedule of Stat-Ease appearances
6. Workshop alert: See when and where to learn about DOE
PS. Quote for the month—Edison's positive view of negative experimental results
Many of you by now may 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 September issue at http://www.statease.com/news/news0409.pdf.
The feature article, "Playing with Paper Helicopters," discusses my experiences with this in-class exercise on design of experiments at South Dakota School of Mines and Technology as well as the Ohio State University.
The other article, by consultant Shari Kraber, details power in terms of experimental runs as a function of signal to noise ratio.
"Using Design Expert® software, I've completed the residual analysis and diagnostics for an experimental design. For the most part, the patterns look good:
- Normal Probability Plot—linear
- Residuals vs Predicted—random scatter
- Residuals vs Run Number—random scatter
- Predicted vs Actual—acceptable
- Box-Cox Plot—no transformation recommended
My concern is that the "Residual vs Factor" plot shows a megaphone shape for two factors. Does this invalidate the ANOVA?"
Answer (from Stat-Ease consultant Pat Whitcomb):
"Without looking at the diagnostics myself, I can't give you a definite answer. However, from what you've described—with so many normal diagnostics, the ANOVA should be fine."
For more details on residual diagnostics see http://www.itl.nist.gov/div898/handbook/pri/section2/pri24.htm from the NIST/Sematech e-Handbook of Statistical Methods. They note that smiles :) or frowns >:( are things to watch for on residual versus multilevel factors. However, ANOVA is robust to moderate departures from the assumption of equal variances so experimenters should concern themselves only over large differences in the scatter of residuals over factor ranges, particularly if other diagnostic plots appear abnormal. In that case, response transformations should be considered.
This would not happen to a competent analyst of experimental data such as the person asking this FAQ, but we've seen just about everything here at Stat-Ease, including models that omit highly-significant main effects. In such case, the plot of residuals versus the omitted factor will show a clear upwards or downward pattern.
(Learn more about diagnosing residuals by attending the three-day computer-intensive workshop Experiment Design Made Easy." See http://www.statease.com/clas_edme.html for a course description. Link from this page to the course outline and schedule. Then, if you like, enroll online.)
Sorry, due to the high cost of shipping, this offer applied only to residents of the United States and Canada, 45 of whom sent me an e-mail that they'd appreciate a free copy of Box and Drapers' classic statistical text "Empirical Model-Building and Response Surfaces." Originally published in 1987 (John Wiley and Sons, New York), this book is a classic in the field of response surface methods (RSM) for process optimization. However, for workshops on this topic,* Stat-Ease now uses Myers and Montgomery's "Response Surface Methods" (see this book and others listed for purchase via ecommerce at http://www.statease.com/prodbook.html).
The winners, chosen at random, are:
—Dan Obermiller, Master Black Belt, Dow Chemical
—Mike Moore, Cabot Microelectronics
—Casey Swete, General Electric
Thank you to all who participated in this drawing. If it's any consolation for those who did not win, Stat-Ease will likely free up more of these textbooks, or others of like quality, in future as they are replaced with newer editions.
*(Learn more about RSM by attending the three-day computer-intensive workshop "Response Surface Methods for Process Optimization." See http://www.statease.com/clas_rsm.html for a complete description. Link from this page to the course outline and schedule. Then, if you like, enroll online.)
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Share a DOE with our writer at: RABURNHAM@PublicationCoordination.com
Rename proprietary factors if needed to maintain confidentiality. A thirty-minute interview, some e-mails, a review, and it's done!
Click on http://www.statease.com/events.html for a list of appearances by Stat-Ease professionals. We hope to see you sometime in the near future!
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.
I hope you learned something from this issue. Address your general questions and comments to me at: Mark@StatEase.com
Mark J. Anderson, PE, CQE
PS. Quote for the month—Edison's positive view of negative experimental results:
"I have not failed. I've just found 10,000 ways that won't work."
—Thomas Alva Edison (1847-1931)
Trademarks: Design-Ease, Design-Expert and Stat-Ease are registered trademarks of Stat-Ease, Inc.
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DOE FAQ Alert—Copyright 2004
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