Issue: Volume 10, Number 8 (Circulation: Over 5500 worldwide)
Date: August 2010
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].

Also, Stat-Ease offers an interactive web site — the Support Forum for Experiment Design at http://forum.statease.com. Anyone (after gaining approval for registration) can post questions and answers to the Forum, which is open for all to see (with moderation). Furthermore, the Forum provides program help for Design-Ease® and Design-Expert® software. Check it out and search for answers. If you come up empty, do not be shy: Ask your question! Also, this being a forum, we encourage you to weigh in!

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):

— Reaching your boiling point
— What value for p is right for testing t (or tasting tea)?
— Ink made to last and fonts that minimize its consumption

* Need a feed or e-mail updates from StatsMadeEasy? Go to http://feeds.feedburner.com/StatsMadeEasy. It's easy! Even simpler: Just enter your e-mail in the new forwarding field and get new StatsMadeEasy blogs delivered directly.

"Your StatsMadeEasy blogs brighten up a dreary workday!"
— Applied Statistician, Florida

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

1. FAQ: What if you see a trend in residuals versus run order?
2. FAQ: Why no propagation of error (POE) on a linear model?
3. Expert FAQ: What does I-V optimal design offer over D optimal?
4. Info alert (5 publications!): DOE scores in a wide range of applications — making engines more powerful, solving a tableting problem, improving a fluid-bed coating process, optimizing an assay and providing quality by design (QbD).
5. Webinar Alert: Encore of DOE Made Easy and More Powerful via Design-Expert Software, Part 2 — Response Surface Methods (RSM) for Process Optimization
6. Events alert: Optimal mixture design of experiments for QbD
7. Workshop alert: See when and where to learn about DOE

P.S. Quote for the month: Dirt on how 5 percent became the critical p value for hypothesis testing.

(Page down to the end of this e-zine to enjoy the actual quote.)

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1. FAQ: What if you see a trend in residuals versus run order?

-----Original Question-----
From: Ink Chemist
"I appreciate your monthly DOE FAQ Alert, which I've been subscribing to for a very long time. I do have a question about residual analysis. Can you tell me what is wrong when the graph of residuals versus response (measured values) shows a correlation? Is the model not good enough and/or is there a factor missing?"

Answer (from Stat-Ease Consultant Wayne Adams):
"The plot you are talking about is the residual vs. run order. I will go over its interpretation.

An obvious pattern in the residuals relative to the run order can indicate there is an time-dependant effect from a lurking variable. These trends can confuse the analysis to the point of completely invalidating the conclusion.

Such a pattern means you should look for something other than a factor that was varying during the experiment. If a cause can be identified, then corrections can be made to the analysis.

Patterns can also happen randomly. In these cases, even after considerable effort to find something, no lurking variable can be identified and no cause for the pattern can be found. Proceed with the analysis and note the discrepancy on the plot in the report."

Consultant Pat Whitcombs adds:
"Over the years residual plots have on numerous occasions led me to vital discoveries of beneficial transformations, invalid runs, and the presence of lurking factors. For example, patterns on the residual versus run order (time) uncovered these lurking factors in two particular cases:

—> Oxidation of an active ingredient in the master batch of raw material that diminished the final yield over time.
—> Sinusoidal behavior that belied worn bearings in an extruder. I continue to be amazed at the wealth of information in residual plots, and it's all free for the taking."

(Learn more about diagnosing residuals by attending the two-day computer-intensive workshop "Experiment Design Made Easy." See http://www.statease.com/clas_edme.html for a description of this class and link from this page to the course outline and schedule. Then, if you like, enroll online.)

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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, #9-10 Oct 09, #9-11 Nov 09, #9-12 Dec 09, #10-1 Jan 10, #10-2 Feb 10, #10-3 Mar 10, #10-4 April 10, #10-5 May 10, #10-6 June 10, #10-7 July 10, #10-8 Aug 10 (see above)

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