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From |
tmmanini <tmmanini@mailbox.syr.edu> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: ancova for repeated designs |

Date |
Sat, 14 Aug 2004 18:04:21 -0400 |

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. id g t y x 1 1 1 1 1 1 1 2 1 1 1 1 3 1 1 3 2 1 1 1 3 2 2 1 1 3 2 3 1 1 4 3 1 5 5 4 3 2 5 5 4 3 3 1 5 6 2 1 5 5 6 2 2 4 5 6 2 3 1 5 7 3 1 6 6 7 3 3 6 6 7 3 2 6 6 8 1 1 3 3 8 1 2 3 3 8 1 3 3 3 9 1 1 6 6 9 1 2 6 6 9 1 3 6 6 10 1 1 1 1 10 1 2 1 1 10 1 3 1 1 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) Number of obs = 24 R-squared = 0.9126 Root MSE = .983192 Adj R-squared = 0.7990 Source Partial SS df MS F Prob > F -----------+---------------------------------------------------- Model 100.958333 13 7.76602564 8.03 0.0012 x 37.5 1 37.5 25.05 0.0075 g 2.98197901 2 1.4909895 1.00 0.4457 idg 5.98844884 4 1.49711221 -----------+---------------------------------------------------- t 7.6 2 3.8 3.93 0.0550 g*t 4.91666667 4 1.22916667 1.27 0.3437 x*g 0 0 Residual 9.66666667 10 .966666667 -----------+---------------------------------------------------- Total 110.625 23 4.80978261 when I try the adjust command, I come up with blanks (below) adjust x, by(g t) Dependent variable: y Command: anova Variable left as is: id Covariate set to mean: x = 3.5 t g 1 2 3 1 2 3 What is really annoying is the fact that I can use this same design in SPSS and not have a problem, but I would really like to use STATA. I've examined all the material on the listserv I can find about this topic, which has lead me to this point, but if you have another resource please point me in the right direction. * * 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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