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From |
"Ploutz-Snyder, Robert (JSC-SK)[USRA]" <robert.ploutz-snyder-1@nasa.gov> |

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

Subject |
RE: st: Unbalanced repeated measures analysis question |

Date |
Thu, 22 Jul 2010 08:35:51 -0500 |

Karin, You could define your model the way you suggested, yes, however mixed models can be specified a number of different ways depending on your research goals and how you want to consider the nesting of your repeated measures factors (i.e. random terms). There are a number of excellent books on this type of analysis, going by names including mixed-effects modeling, mixed modeling, higher level modeling (HLM), multi-level modeling (MLM) and probably a few other terms... If you are interested in a more Applied book that uses Stata in particular, Rabe-Hesketh and Skrondal put together a nice one book called Multilevel and Longitudinal Modeling Using Stata. I think you might do well to take a course in MLM if you can to at least wrap your brain around the theory. But if you want to jump right in then a book like this one could get you going in the right direction. Rob -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of K Jensen Sent: Thursday, July 22, 2010 5:33 AM To: statalist@hsphsun2.harvard.edu Subject: Re: st: Unbalanced repeated measures analysis question Thanks to Robert and David for your helpful comments. Sorry to sound stupid here but mixed models are entirely new to me. I have been reading up on them. I have the variables outlined below: SubjectID MeasurerID MeasurerType Result GoldStandard where MeasurerID is always a certain MeasurerType (1-3) SubjectID and MeasurerID should be random effects and MeasurerType fixed? How would you specify that in the xtmixed syntax? I am confused about having two grouping variables for the random effects. Karin On 21 July 2010 22:37, Ploutz-Snyder, Robert (JSC-SK)[USRA] <robert.ploutz-snyder-1@nasa.gov> wrote: > Karin, > I feel your pain RE Stata's anova syntax for repeated measures... > But I also agree with David that I think your better bet is > probably to use -xtmixed- and then apply -margins- for your > post-hoc comparisons, given the imbalance issue. You can use > -margins- to compare each of the three measures to the gold > standard--akin to simple effect contrasts. > > If you wish to remain in the anova syntax, you might want to > check out the user written -anovalator- command, thanks to Phil > Ender from UCLA. But from the sounds of your imbalanced design, > I would tend to lean more to -xtmixed- with -margins- > > (BTW--the Phil's website at UCLA has some nice walk-throughs > of all of this.) > > Rob > > -----Original Message----- > From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Airey, David C > Sent: Wednesday, July 21, 2010 4:07 PM > To: statalist@hsphsun2.harvard.edu > Subject: re: st: Unbalanced repeated measures analysis question > > > I think when you have comparisons to a gold standard, or all > comparisons with one control, that there are specific ANOVA > post-hoc tests that perform better than all possible or all > pairwise comparisons procedures. > > There is the complication that you are testing for equivalence, > as you say. > > The Stata command -xtmixed- can do what -anova- can. Sometimes > -manova- or -mvtest- is useful with repeated measures too. > > It is hard to understand how your design is unbalanced without > seeing the data cross-tabs, etc. > >> Hi >> >> I have data on measuring a biological property for three different >> methods plus a gold standard. Different people were trained in each >> method (1,2 or 3) and measured the same subjects during different >> sessions, together with the gold standard measurement. >> >> So the data look like >> SubjectID MeasurerID MeasurerType Result GoldStandard Diff >> 1 1 1 95 99 -4 >> 1 2 3 102 99 +3 >> 1 3 2 92 99 -7 >> ... >> 1 10 3 105 99 +6 >> 2 1 3 98 100 -2 >> ... >> >> Sometimes patients would be called in to see the consultant and so >> missed for a particular measurer, but otherwise all the measurers >> would measure all the patients seen in a particular session. Different >> sets of measurers (but all trained by methods 1,2 or 3) were used on >> each session (individual measurers 1-10 on session 1, 11-20 on session >> 2 etc). >> >> The gold standard measurements on each session are roughly normally >> distributed, as are the differences from the gold standard. We are >> interested in the accuracy of each of the three methods. >> >> Is it OK to do some sort of repeated measures ANOVA here, with an >> unbalanced design? If it is what would be the syntax (Stata 10)? Sorry >> to sound pathetic but I just can't get the anova command with the >> repeated option to work here. >> >> Is there a better measure to use than the difference to reflect the >> fact that we are interested in a comparison with a gold standard? >> >> Thankyou >> Karin * * 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/ * * 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/

**Follow-Ups**:**RE: st: Unbalanced repeated measures analysis question***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**Re: st: Unbalanced repeated measures analysis question***From:*K Jensen <k.x.jensen@gmail.com>

**References**:**re: st: Unbalanced repeated measures analysis question***From:*"Airey, David C" <david.airey@Vanderbilt.Edu>

**RE: st: Unbalanced repeated measures analysis question***From:*"Ploutz-Snyder, Robert (JSC-SK)[USRA]" <robert.ploutz-snyder-1@nasa.gov>

**Re: st: Unbalanced repeated measures analysis question***From:*K Jensen <k.x.jensen@gmail.com>

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