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From |
David Airey <david.airey@Vanderbilt.Edu> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Repeated Measures Manova contrasts |

Date |
Tue, 23 Oct 2007 13:53:55 -0500 |

. /* manova is only well documented in the manual...with many examples and a reference to a text, Rencher (2002). I think Phil Ender will talk about repeated measures possibilities in Stata soon at the West Coast SUG. You should get the manuals or borrow to read the sections you need. Here's some code from a relevant example. Would be good to have FAQ on repeated measures expanded to include this aspect of MANOVA. 1 repeated measures variable and 3 grouping variables allow period and dependent variable interaction by use of ytransform() option of manovatest postestimation command see example 10 in [MV], code lifted here see example 11 in [MV] for multiple dependent variables A good basic level text I like to MANOVA is Maxwell and Delaney. */ use http://www.stata-press.com/data/r10/fabric, clear describe quietly manova y1 y2 y3 = p t p*t f p*f t*f p*t*f /* Create matrix c with rows corresponding to the linear and quadratic contrasts for the three dependent variables; 3 columns for each y, first row linear contrast, second row quadratic contrast. Use matrix c for test of period and dv interactions */ matrix c = (-1, 0, 1 \ -1, 2, -1) /* tests of interaction of period with RHS terms... */ manovatest p, ytransform(c) manovatest t, ytransform(c) manovatest p*t, ytransform(c) manovatest f, ytransform(c) manovatest p*f, ytransform(c) manovatest t*f, ytransform(c) manovatest p*t*f, ytransform(c) /* testing main effects of period... */ manovatest, showorder // use to help make the m matrix below /* [MV] explanation: ...create a row vector m based on above that has: 1 for first column for constant 1/3 for 3 columns for p 1/2 for 2 columns for t 1/2 for 2 columns for f 1/6 for 6 columns for p*t 1/6 for 6 columns for p*f 1/4 for 4 columns for t*f 1/12 for 12 columns for p*t*f */ matrix m = (1), /// J(1,3,1/3), /// J(1,2,1/2), /// J(1,2,1/2), /// J(1,6,1/6), /// J(1,6,1/6), /// J(1,4,1/4), /// J(1,12,1/12) manovatest, test(m) ytransform(c) On Oct 23, 2007, at 11:51 AM, A Noyb wrote:

I ordinarily use SPSS, and am new to Stata; this is my first post to this list. Could anyone refer me to resources for understanding the matrix command, more particularly for _laying out the contrast coefficients depending on the number of levels of the IV and DV in MANOVA_? I hope my questions are proper ones for this list and are not too basic. I am doing a Manova and am most interested in the time by treatment interaction. I am baffled by the contrasts described in the worked example at http://www.ats.ucla.edu/stat/stata/faq/manovarep.htm . Any help in understanding how to write contrasts in such a situation would be much appreciated. The online Stata manual doesn't provide much guidance nor have other web resources/our university library. Thank you, Sandy Below is a more detailed description of the questions I have about the worked example, and also about my own data, in case anyone is inclined/willing to give their two cents. WORKED EXAMPLE input s a y1 y2 y3 y4 1 1 3 4 7 7 2 1 6 5 8 8 3 1 3 4 7 9 4 1 3 3 6 8 5 2 1 2 5 10 6 2 2 3 6 10 7 2 2 4 5 9 8 2 2 3 6 11 end manova y1 y2 y3 y4 = a mat ymat = (1,0,0,-1\0,1,0,-1\0,0,1,-1) mat list ymat I don't understand how these coefficients measure the interaction, it looks like they just compare each of y1 to y3 in turn to y4. MY DATA My dataset has three repeat measurements of a number of variables and several IVs of differing levels. DVTime1 DVTime2 DVTime3 IV1 IV2 3 4 3 1 1 1 2 4 1 2 2 1 5 1 3 3 4 6 1 1 3 5 4 2 2 2 4 3 2 4 3 1 5 2 2 1 4 4 2 2 3 4 4 2 1 The main analysis I need to do looks at the interaction of time and IV1, which has 2 levels. I created a matrix using the following Mat ymat = (1,0,-1\0,1,-1) However the output was quite different from the output from SPSS Manova for the very same data and analysis. Since I am more comfortable with SPSS, I thought it prudent to check my work before proceeding too far. What should the matrix be to look at the time by treatment interaction? The reason I am not using SPSS to do these analyses is that Stata is better suited to my non-normal data. (I ultimately plan to use the negative binomial option on the Manova, once I get the bugs worked out.) Thank you very much for any help you can give. __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com * * 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/

-- David C. Airey, Ph.D. Pharmacology Research Assistant Professor Center for Human Genetics Research Member Department of Pharmacology School of Medicine Vanderbilt University Rm 8158A Bldg MR3 465 21st Avenue South Nashville, TN 37232-8548 TEL (615) 936-1510 FAX (615) 936-3747 EMAIL david.airey@vanderbilt.edu URL http://people.vanderbilt.edu/~david.c.airey/dca_cv.pdf URL http://www.vanderbilt.edu/pharmacology * * 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/

**References**:**st: Repeated Measures Manova contrasts***From:*A Noyb <kakamega2005@yahoo.com>

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