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

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

Subject |
st: Re: ancova for repeated designs |

Date |
Sun, 15 Aug 2004 13:08:00 +0900 |

tmmanini wrote: I'm trying to run a supposed simple ancova model, controlling for pre-test values. subjects are placed in three groups and then tested at three different time points. I would like to covary for the first time point, examine the group*covariate interaction and calculate the adjusted means. I've listed 10 subjects (I really have 32) below (g=group, t=time, y=depvar, x=covariate). g is a between factor, t is a repeated factor. [excerpted] When I run this with the 10 subjects listed above, I can't get the interaction term b/c the df=0 (see below) anova y x g / id | g t g*t x*g, cont(x) rep(t) [excerpted] when I try the adjust command, I come up with blanks (below) adjust x, by(g t) -------------------------------------------------------------------------------- It seems that you have used the response (dependent) variable's value at the first timepoint (t ==1) as the covariate, x, and this seems to be what you mean by "covary for the first time point." If so, I imagine that you'd want to include -if t > 1-. Anyway, there is no variation of x within id, so there won't be any within the id|g error term, either, and it should be put to the right of the id|g random error term. Repeated-measures ANOVA for time-invarying (and time-varying) covariates are discussed in B. J. Winer, D. R. Brown, and K. M. Michels. _ Statistical Principles in Experimental Design _ Third Edition. (New York: McGraw-Hill, 1991). I don't have my copy at-hand, but try something like -anova y g / id|g x x*g t g*t if t > 1, repeated(t) continuous(x)- . Double-check with Winer on this. As for predicted values, id is in the list of regressors in -anova-, so you'd need to include it in the list of -by()- variables and then limit the values to three that represent the three groups, as follows: adjust x if id<=4, by(g t id) Joseph Coveney P.S. It seems that y takes on only six values, and a plurality is 1. Consider methods for limited dependent values, such as -reoprob- or -gllamm-. * * 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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