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From |
khigbee@stata.com |

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Fri, 22 Aug 2003 17:48:22 -0500 |

David Airey <david.airey@vanderbilt.edu> asks: > I guess I would have expected the command syntax to then be: > > . anova score noise/subject|noise period/period*subject|noise > noise*period/perio > > d*subject|noise dial/dial*subject|noise noise*dial/dial*subject|noise > period*d > > ial noise*period*dial, repeated(period dial) > repeated term > r(141); > > which fails, > > rather than: > > . anova score noise / subject|noise period noise*period / > period*subject|noise > dial noise*dial / dial*subject|noise period*dial noise*period*dial , > repeated(period dial) > > I thought when a term did not specify the denominator, the residual was > used. In the latter command, the terms period and dial are listed > without a denominator so I thought they were using the residual. But > when I look at the output, it's clear they are using a different (and > correct) denominator. What am I not understanding in telling or not > telling Stata what I want? And then David says: > OK. > > Now I think I understand the syntax, and why Ken's command does what it > correctly does. All terms prior to the slash without specified > denominators are tested with the term after the slash. This is a little > cryptic and depends on correctly grouping terms tested with the same > denominator when issuing an ANOVA command. But it makes sense once you > see it. I think the slash is like a shorthand for the lines used to > separate the ANOVA tables > > ANOVA y a b c/d tests each of a, b, and c, over d. > ANOVA y a/ b c/d tests a over error, and b and c over d > ANOVA y a/d b/d c/d is disallowed syntax > > Right? "[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. > 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. 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. Ken Higbee khigbee@stata.com StataCorp 1-800-STATAPC * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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