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From |
khigbee@stata.com |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: repeated measures ANOVA with missing observations |

Date |
Mon, 02 Dec 2002 12:34:25 -0600 |

David Ronis <dronis@umich.edu> asks: > My experience with other software is that repeated measures ANOVA will > either drop cases with any missing data or fail to run when there are > missing data. Before getting into more complex procedures like SAS PROC > MIXED I thought I'd give it a try in Stata. My expectation was that the > failure would help motivate me for the work ahead. > > I studied Kenneth Higbee's FAQ a bit at > www.stata.com/support/faqs/stat/anova2.html > > <cut> > > To my surprise, the following code ran and gave results that seemed > reasonable (given my eyeballing the data and means): > > clear > capture log close > log using preeval2.log, replace > * Approach from Higbee FAQ > * http://www.stata.com/support/faqs/stat/anova2.html > set matsize 800 > set memory 4m > set more off > use e:\yeo2\vo2-pree-val.dta > > anova vo2 id time / time*id stage / stage*id machine / machine*id /* > */ time*stage / time*stage*id /* > */ time*machine / time*machine*id /* > */ stage*machine / stage*machine*id /* > */ time*stage*machine / , /* > */ repeated (time stage machine) > > I'm wondering whether this is really an appropriate analysis, and what > assumptions it / I may be making (especially unusual ones)? For sig test > results I'm looking at the adjusted ones, not those in the initial ANOVA > table. It has been about 20 years since I studied ANOVA. In the interest of brevity I will point you (and others interested in the subject) back to some statalist threads of long ago. At the end of July 2001 a similar question was asked. You can go to http://groups.yahoo.com/group/statalist/message/25690 to read the message. It quotes from Milliken & Johnson, 1984, "Analysis of Messy Data, Volume 1: Designed Experiments", Van Nostrand Reinhold Company, New York. ISBN: 0-534-02713-7 and has some further discussion. It also points to a statalist discussion that happened in mid October of 2000. I would point you to a web link, but the Yahoo site keeps only a certain size buffer of old messages (currently you can only go back to Nov. of 2000). The archives at Harvard http://www.hsph.harvard.edu/cgi-bin/lwgate/STATALIST/archives/ appear to only go back to January 2001. And the archives at http://www.uc.pt/pessoal/ramalheira/stblist.htm appear to go from 1994 through July 1998. So, since you might have a hard time finding the discussion from Oct. 2000, here is what I wrote on 12 Oct 2000 at the conclusion of the discussion. ----------------------------------------------------------------------- From: khigbee@stata.com To: statalist@hsphsun2.harvard.edu Subject: Re: Unbalanced Repeated Measures ANOVA Al Feiveson <alan.h.feiveson1@jsc.nasa.gov> provides a good caution regarding an example I gave with a significant amount of missing cells in a repeated measures ANOVA. > Ken - I see that Stata will produce the ANOVA as you have > indicated - but how good are the "F" statistics? If I am not > mistaken, they won't really have an exact "F"-distribution even > if the error terms are independent normal and homoscedastic. Of > course, nothing is really normally distributed, etc, anyway, so > this is probably a moot issue. With complicated ANOVA designs having missing cells the "proper" F-tests can be difficult (sometimes impossible) to construct. In simpler designs where residual error is the only error term, the missing cells in the design do not change the use of MSerror for the denominator of the F test. In more complicated designs where there are different error terms for various levels of the model, the expected mean squares in the presence of missing cells can lead to very complicated tests. A discussion of this can be found in Milliken and Johnson, 1984, "Analysis of Messy Data, Volume 1: Designed Experiments", New York: Van Nostrand Reinhold Company In particular, around page 395 it shows an example of how you would form a test, and, as Al Feiveson alludes to, even the test they contrive does not truely follow an F distribution. They say concerning the test that it "... does not have an exact F-distribution since (1) the statistic in the denominator does not have a distribution that is proportional to an exact chi-square distribution, and (2) the numerator and denominator may not be independently distributed. ..." So if you have missing cells in a complicated ANOVA design you will need to exercise caution in interpretting F-tests using non residual error terms. In practice, I believe many people close their eyes tight and proceed with the tests as if the missing cells were not present. With only a small percentage of the cells missing this may be reasonable. With a larger percentage of cells missing it may not be reasonable. ----------------------------------------------------------------------- Ken Higbee khigbee@stata.com STATA CORP 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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