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From |
Philip Ender <ender97@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: xtmixed for repeated measures |

Date |
Mon, 14 Nov 2011 08:39:21 -0800 |

Carrie Petrucci <cpetrucci@emt.org wrote ... I hope I have not offended by including syntax from the other statistical programs, but it seemed like the easiest way to illustrate what I'm trying to do. 1. My dataset is long with up to 3 records per person to represent standardized assessments at intake, 3 months, and 6 months; I wish to look at the pattern of change over time and want to be able to look at data for all participants, rather than just those with data at all 3 time points; these are periodic data runs that I do for project staff for whom I do the evaluation work, and they find it quite motivating to hear where there are trends towards improvement. 2. I "xtset" the data, although I'm unsure this was necessary 3. I used the syntax below thus far and it rendered the model output (I've not yet included a between subjects factor as the SAS and SPSS examples above do): xtmixed bcs xobs || xpid: "bcs" is the dependent variable measured at 3 time points; "xobs" is 1, 3, or 6 to represent intake, 3 months or 6 months; "xpid" is the individual id for each participant 4. I've also run it this way: xtmixed bcs i.xobs || xpid: I assume, but wish to confirm, that both models are valid, with the "i.xobs" incorporating the dummy coding that tests each level of xobs to the first level, which in this case, is intake, which would be an appropriate for this data 5. From there, I cannot figure out the appropriate commands to get the remaining output and to apply a first order autoregression model, which appears to be appropriate for my data. Output in the SAS output that would be helpful to see includes: (These are titles of tables from the SAS output) Estimated Correlation Matrix for Groups Covariance Parameter Estimates AIC and BIC fit statistics Type 3 Tests of Fixed Effects I hope these are easy questions to someone familiar with Stata. Thank you for any assistance you can provide. ---------------------------- Dear Carrie, Here are my suggestions of how to run a repeated measures analysis with -xtmixed-. I note that both the SPSS and SAS examples include a between subjects factor and your code does not. I will stick with your example. . xtmixed bcs i.xobs || xpid:, var reml residuals(ar1, t(visit)) // note the -reml- and -residuals()- options . contrast xobs // anova-type summary table . margins xobs, asbalanced // cell means equivalent to spas emmeans . estat ic // AIC and BIC . pwcompare xobs, mcompare(bonferroni) // pairwise post-hoc if needed (wanted) I hope this is useful. -- Phil Ender UCLA Statistical Consulting Group * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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