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st: Specifying a repeated measures analysis in anova and xtmixed

From   David Pieczkiewicz <>
Subject   st: Specifying a repeated measures analysis in anova and xtmixed
Date   Mon, 1 Aug 2005 13:59:44 -0500


I am relatively new to Stata (and to the types of analyses I ask about below), and I hope my questions are not too elementary for the list. I've consulted the manuals and what online help I could find, but I was unable to resolve my issues. I'm hoping some of you can offer pointers.

Briefly, I am conducting research dealing with different ways of presenting medical data to clinicians. One aspect I am interested in is the time it takes for clinicians to reach decisions when using these displays, and whether different ways of presenting the same data take longer or shorter times than others.

My experimental design consists of a series of cases (i.e., data taken from individual patients), each shown in different display formats (i.e., modalities), to a variety of clinician readers, who then decide on what to do with that patient. *Every* case is shown in *every* modality to *every* reader. If I'm using the correct terminology, this is a complete, balanced design, with no nesting, blocking, etc. For each reader, the order of cases/modalities is randomized. Different data are collected from each combination of case, modality, and reader, including the time it takes for decisions to be rendered in each case/modality/reader combination.

My understanding is that this is a repeated measures design, with cases being repeated over both modality and reader. I have two questions, dealing with how to properly analyze this data with Stata (version 9):

1) Using the anova command, I believe this is an appropriate statement to examine the influence of modality on decision time:

anova lntime case modality reader modality*reader modality*case reader*case, repeated(modality reader)

where "lntime" is the natural log-transformed decision time(s), and case, modality and reader are the categorical variables as described above. While I am most interested in the factor "modality," I have some interest in the first-order interactions, hence their inclusion here.

Does this command look correct? Part of my uncertainty was that for a while, I was under the impression that the variables going into the repeated() clause were the experimental units being repeated (here, cases). I've come to understand that repeated() should instead contain the variables that the experimental units are being repeated over. I want to make sure I'm accurate in this before I get too far in.

2) If I wanted to analyze this as a mixed model, what is an appropriate way to use xtmixed? I consider case and reader to be random effects, as both are drawn from larger populations of possible cases and readers. Likewise, I consider modality to be a fixed factor, as there are only N modalities of interest in any particular phase of my study. As such, I came up with the following as a rough guess of how to express it:

xtmixed lntime modality || case: || reader:

Given that this is a repeated measures design, how can/should I express the fact that the cases are being repeated over all levels of modality and reader? I'm much less certain about this one, mainly because I have even less practical experience with mixed models than ANOVAs.

Any pointers, hints, clarifications, anyone can provide will be well- appreciated, not just for my understanding of how to examine this particular experiment, but repeated measures and mixed model analyses in general. Thank you!


David S. Pieczkiewicz, M.A. (612) 625-3241
Division of Health Informatics
University of Minnesota, Twin Cities Campus

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