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From | Philip Jones <pjones8@uwo.ca> |
To | statalist@hsphsun2.harvard.edu |
Subject | st: Analysis of experiment involving baseline measurements |
Date | Tue, 28 Jun 2011 05:31:28 -0400 |
Hello, I am looking for some guidance from Statalisters with respect to analysis of a randomized trial. Please forgive me in advance for the length of this message! The trial's intervention is a teaching technique. Participants are randomly allocated to the new technique (simulation) or the old (didactic group). The dependent variable is continuous on a scale from 0-24 (the sum of numeric responses on a questionnaire), and is measured at baseline ('pre') and twice more (immediately after the intervention and after six weeks: 'post' and 'sixwk'). I have set up my data file as follows (this sample data looks normal in a fixed-width font): +---------------------------------+ | group pre post sixwk | |---------------------------------| 1. | didactic 14 24 17 | 2. | simulation 12 23 22 | 3. | didactic 18 24 19 | 4. | simulation 16 23 23 | 5. | didactic 10 20 16 | |---------------------------------| 6. | simulation 9 21 24 | 7. | didactic 14 22 20 | 8. | simulation 14 23 16 | +---------------------------------+ According to Senn (1), if there were only one measurement post-baseline measurement, the preferred way of analyzing this data would be to compare the two groups on the 'post' score while adjusting for the baseline values using ANCOVA. However, I have two post-baseline measurements, and I'm wondering how I might be able to take the timing of the measurements into account using a mixed model, or whether I should bother getting that technical and instead perform two separate comparisons: 1) 'post' result adjusted for baseline 2) 'sixwk' result adjusted for baseline These analyses are easy enough: regress sixwk i.group c.pre Has anyone got an opinion as to whether or not I should go down the road of a mixed ANOVA to analyze these data? Many thanks in advance for any advice/assistance! Phil (1) Senn S, Statistical Issues in Drug Development, 2nd ed. * * 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/