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From |
David Airey <david.airey@vanderbilt.edu> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: SP/RM[1,2] EMS |

Date |
Sat, 23 Aug 2003 11:18:13 -0500 |

Ken kindly took the time to clarify problems in my understanding which have more to do with my statistical ineptness than any problems with the manuals or FAQs:

If I can do better I'll send something! I'll eventually share my data for the FAQ to include SP/RM[2,2], SP/RM[2,3], and SP/RM[2,4] ANOVAs."[R] anova" page 43 (Version 8 manual) discusses the "/" notation. I admit that there may be better ways to word that discussion. If anyone has some candidate better wording, send it my way. I had hoped that the various nested and mixed models that followed in "[R] anova" would have been enough to clarify.

For your 3 examples above: > ANOVA y a b c/d tests each of a, b, and c, over d. Yes. > ANOVA y a/ b c/d tests a over error, and b and c over d No. a is tested by b. b is not tested. c is tested by d.

Ah!

None of my texts discuss "standard order". I assume this means ordered by the same denominator. I don't have the Winer text. Yes, I should buy it. Instead, I have an introductory text by George Cobb, which I like.> ANOVA y a/d b/d c/d is disallowed syntax It is disallowed because the term "d" appears multiple times in the model, not because of the slashes. First and foremost the -anova- command is supposed to indicate which terms are in the model. Secondarily, the "/" notation is helpful in getting many (often all) of the tests of interest when the model is written in standard order.

You can get any other tests you want using -test-. When you test terms you can use the "/" to indicate the error term. See the examples on pages 60-62 of "[R] anova". For an example of this in a complicated ANOVA see http://www.stata.com/support/faqs/stat/anova2.html#expand911 which is one example in a large FAQ filled with examples. In this case, the "/" in -anova- obtained all but 3 of the tests of interest. The -test- command using "/" was used to get those 3. The -anova- command that was typed for that example was: anova res A / G|A B B*A / B*G|A / S|B*G|A C C*A / C*G|A C*B C*B*A / C*B*G|A / C*S|B*G|A D D*A / D*G|A D*B D*B*A / D*B*G|A / D*S|B*G|A D*C D*C*A / D*C*G|A D*C*B D*C*B*A / D*C*B*G|A / , repeated(C D) Imagine if instead of using the current "/" notation you had to specify the error term for each instance. It would be even more complicated, and I contend even harder to understand.

I totally agree.

The facility to use both is very convenient. I look forward to using your very cool ANOVA multiple test additions too!One approach, if you dislike the "/" notation in -anova- is to not use the "/", and instead do all tests of interest using the -test- command after -anova-. That way, one by one you can specify the term to be tested and the error term for that test. Once you get used to the "/" notation, I am betting you will want to use it again.

With much customer satisfaction,

-Dave

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