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From |
Joseph Coveney <jcoveney@bigplanet.com> |

To |
Statalist <statalist@hsphsun2.harvard.edu> |

Subject |
Re: st: re: ANCOVA for pre post designs |

Date |
Mon, 05 Jan 2004 13:32:46 +0900 |

David Airey posted last week: ---------------------------------------------------------------------------- > OK, let me ask a simpler question: can one have baseline covariates in > within-subjects ANOVAs like we have in ANCOVAs, which are > between-subject ANOVAs but with covariates? Just to close out my thread, I asked if the ANOVA options, continuous() and repeated(), could be used in a repeated measures ANCOVA, a model discussed by Winer from looking at the online TOC. Stata's response was that I look at MANOVA instead. So I guess not, but good enough. ---------------------------------------------------------------------------- I believe that you *can* use the -continouous()- and -repeated()- options together. With involved models, you might need to explicitly specify the between-subjects error term using the -bse()- option, but that's not any impediment. I've illustrated their combined use in the do-file below using the two repeated-measures ANCOVA examples from Chapter 10 of B. J. Winer, D. R. Brown & K. M. Michels, _Statistical Principles in Experimental Design_ 3rd Edition. (NY: McGraw-Hill, 1991), pp. 820-36. The first example has a single time-invariate covariate for each individual; the second has a time-varying covariate (i.e., the covariate has a different value at each repeated measurement). Note that the two examples in Winer, Brown & Michels don't need the -repeated()- option, since they both have only two observations. I'm not sure why Stata Corp. recommended considering -manova- to David, but perhaps it's because of the admonition stated in the preface to the repeated-measures ANCOVA section from Winer, Brown & Michels (Section 10.5, p. 820), "In the behavioral sciences area, the covariance matrix for the repeated measures is very likely to be more complicated than the corresponding covariance structure assumed for the usual split-plot design [in agricultural research]." David's original posting related to the area of schizophrenia and sensorimotor inhibition. Joseph Coveney ---------------------------------------------------------------------------- clear set more off * * Table 10.34 on Page 829 (single covariate for all observations) * input byte a byte x byte y1 byte y2 1 3 5 8 1 5 15 12 1 8 20 14 1 2 12 6 2 1 15 10 2 8 25 20 2 10 20 15 2 2 15 10 end generate byte individual = _n * Note that Table 10.34 has a typographical error, which is corrected below. replace y1 = 10 in 1 reshape long y, i(individual) j(b) * ANOVA table (Table 10.35 top, p. 830; note rounding errors in the book) anova y a / individual | a b a*b * ANCOVA table (Table 10.35 bottom) anova y x a / individual | a b a*b, continuous(x) repeated(b) * * Table 10.36 on Page 833 (distinct covariate value for each observation) * clear input byte a byte x1 byte y1 byte x2 byte y2 1 3 8 4 14 1 5 11 9 18 1 11 16 14 22 2 2 6 1 8 2 8 12 9 14 2 10 9 9 10 3 7 10 4 10 3 8 14 10 18 3 9 15 12 22 end generate byte individual = _n egen byte x_sum = rsum(x1 x2) reshape long x y, i(individual) j(b) * ANOVA table (Table 10.37) anova y a / individual | a b a*b * ANCOVA table (Table 10.38) Here, Stata balks if -bse()- isn't specified. anova y x_sum a / individual | a x b a*b, continuous(x x_sum) /// repeated(b) bse(individual | a) exit * * 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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