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From |
khigbee@stata.com |

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Tue, 26 Aug 2003 07:53:51 -0500 |

David Airey <david.airey@vanderbilt.edu> has a large repeated measures ANOVA that he is designing, and he is hitting the 11,000 matrix size limit in Stata/SE (The limit is 800 in Intercooled Stata). He mentions that he could assume higher order interactions are not significant and drop them from the model to fall under the limit. He then asks if anyone has other design suggestions. Here is part of a message I have just sent to David: Another possibility you might want to explore is to abandon repeated measures ANOVA and move over to using MANOVA. You can see a discussion of this in "[R] manova" in the section titled "Multivariate analysis of variance with repeated measures" on pages 395--407. In this case what you are doing is modeling the repeated measures as the dependent variables in the MANOVA. The -manovatest- command after -manova- provides the mechanism for getting at various tests of terms of interest involving linear combinations of the various dependent variables and also of the remaining terms in the model. I have not analyzed your setup to see if MANOVA will work well for you. What can happen is that depending on your setup, you might not have enough observations for the MANOVA to be estimated. That is one of the possible disadvantages to MANOVA versus repeated measures ANOVA. On the flip side, there are some important advantages to MANOVA over repeated measures ANOVA 1. Since all the repeated variables are represented as dependent variables, you are not forming so many large interactions and your resulting X'X matrix is smaller. (You are more likely to be able to fit within the matrix size limitiation.) 2. The -manova- approach directly models the repeated measures aspect of the design (it allows correlations between the dependent variables) instead of trying to correct for it as is done in repeated measures ANOVA (the various epsilon correction factors that are applied in rep. measures ANOVA). 3. Some argue that you should always use MANOVA over repeated measures ANOVA (because of #2) unless you do not have enough observations (when the data are wide -- see discussion below). The data structure used in MANOVA versus ANOVA is wide versus long (think -reshape- command). So you would use -reshape- (maybe multiple times since there are several repeated measures variables in your design) to get to a wide dataset. 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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