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

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

Subject |
Re: st: Specifying a repeated measures analysis in anova and xtmixed |

Date |
Tue, 02 Aug 2005 14:35:13 +0900 |

David Pieczkiewicz asked about setting up a mixed model with -anova- and -xtmixed-. There are two random effects (reader and case, which are crossed) and one fixed effect (modality). The dataset is balanced as to representation at each level of fixed and random effects. First excerpt: "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)" If reader is random, then I doubt that you want to include it in the -repeated()- option. It's a little tricky specifying the error terms when setting up -anova- with mixed models. Take a look at Chapter 7 ("Multifactor Experiments Having Repeated Measures on the Same Elements") of B. J. Winer, D. R. Brown and K. M. Michels, _Statistical Principles in Experimental Design_ Third Edition. (New York: McGraw-Hill, 1991). I'm not sure if it has exactly your design, but it might be a start. One of the SAS books-by-users might be helpful, here, too, e.g., R. C. Littel, G. A. Milliken, W. W. Stroup & R. D. Wolfinger, _SAS System for Mixed Models_ (Cary: SAS Institute, 1996); R. C. Littel, W. W. Stroup & R. J. Freund, _SAS for Linear Models_ (2002). Diehl Hall might have copies of them on the monograph shelves. If not, try thumbing though some issues of _Investigative Radiology_ in the periodicals; the study design sounds exactly like something that it'd have a tutorial on. Second excerpt: "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:" There's an example of syntax with crossed random effects in the help file for -xtmixed-. With a fixed effect in addition, I think that the syntax would look more like that in the do-file below. Joseph Coveney clear set more off set seed `=date("2005-08-02", "ymd")' tempfile tmpfil0 set obs 20 generate byte patient_nr = _n generate float patient_effect = invnorm(uniform()) save `tmpfil0', replace clear set obs 20 generate byte physician_nr = _n generate float physician_effect = invnorm(uniform()) cross using `tmpfil0' erase `tmpfil0' forvalues i = 1/3 { generate float log_latency`i' = `i' / 40 + /// physician_effect + 2 * patient_effect + /// invnorm(uniform()) / 3 } reshape long log_latency, i(patient_nr physician_nr) /// j(display_method) xi: xtmixed log_latency i.display_method || /// _all: R.patient_nr || _all: R.physician_nr, nolrtest variance exit * * 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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