```Date: Wed, 8 Jun 2011 12:26:51 -0400 Reply-To: R B Sender: "SPSSX(r) Discussion" From: R B Subject: Re: Mixed Effects Model for group-randomised trial In-Reply-To: <1307484646426-4463258.post@n5.nabble.com> Content-Type: text/plain; charset=ISO-8859-1 Still, Bruce, I think you raise an important point regarding an UNstructured covariance type, particularly with regard to handling repeated measures. I can't remember if there are more than two time points and/or if the OP thinks the residual variance may be different depending on the condition and/or time point. If yes to either of these points, then he/she should consider replacing the second RANDOM statement with a REPEATED statement and make the appropriate specifications. Typically, the RANDOM intercept statement is equivalent to a REPEATED statement with a compound symmetric (CS) structure. If a CS structure is too restrictive, then a REPEATED statement should be considered in place of the second RANDOM statement. Ryan On Tue, Jun 7, 2011 at 6:10 PM, Bruce Weaver wrote: > Absolutely right, Ryan. Good catch. If I'd looked at some of my own MIXED > examples, I would have been reminded that I usually don't even bother > including COVTYPE() when there is a random intercept only. > > > > R B wrote: >> >> Bruce, >> >> Generally speaking, there is no difference between the following two >> RANDOM statements: >> >> /RANDOM=INTERCEPT | SUBJECT(Schoolnumb) COVTYPE(UN) >> /RANDOM=INTERCEPT | SUBJECT(Schoolnumb) COVTYPE(VC) >> >> The UNstructured specification is not necessary if all that is being >> estimated is a RANDOM intercept. Put another way, the intercept has >> nothing with which to covary, so there is no need to specify a >> covariance structure which allows for such covariance. Now, if we were >> estimating both a RANDOM intercept and slope, such as: >> >> /RANDOM=INTERCEPT x | SUBJECT(Schoolnumb) COVTYPE(UN) >> >> then the UNstructured covariance matrix may be appropriate. >> >> Ryan >> >> On Tue, Jun 7, 2011 at 8:44 AM, Bruce Weaver >> wrote: >>> I have a couple comments/questions. >>> >>> 1. Why are you not including Sex as a factor in the model, especially >>> since >>> you are concerned about some imbalance? >>> >>> 2. Some authors (e.g. Twisk) recommend always starting with an >>> unspecified >>> covariance structure. After inspecting the unconstrained covariance >>> matrix >>> this produces, you may be able to impose some other structure (which will >>> save you some degrees of freedom). >>> >>> 3. Have you centered Age on some convenient in-range value (e.g., a value >>> near the minimum, or the mean)? This will make interpretation easier, >>> particularly for the intercept. >>> >>> >>> So I think your model would be something like: >>> >>> MIXED DV BY Condition Time Sex WITH Age >>> /FIXED=Condition Time Sex Age >>> Condition*Time Condition*Sex Time*Sex Condition*Time*Sex | >>> SSTYPE(3) >>> /METHOD=REML >>> /PRINT=SOLUTION TESTCOV >>> /RANDOM=INTERCEPT | SUBJECT(Schoolnumb) COVTYPE(UN) >>> /RANDOM=INTERCEPT | SUBJECT(Student*Schoolnumb) COVTYPE(UN) >>> /EMMEANS=TABLES(Condition) >>> /EMMEANS=TABLES(Time) >>> /EMMEANS=TABLES(Sex) >>> /EMMEANS=TABLES(Condition*Time) >>> /EMMEANS=TABLES(Condition*Sex) >>> /EMMEANS=TABLES(Time*Sex) >>> /EMMEANS=TABLES(Condition*Time*Sex) >>> . >>> >>> >>> HTH. >>> >>> >>> >>> christiane wrote: >>>> >>>> Thank you for replying. I am quite stuck with this one! >>>> >>>> >>>> "Are the child and parent outcome variables the same? If so, then child >>>> v >>>> parent is just another explanatory variable in the model, right? If >>>> not, >>>> I think you'll need two models, one for children and one for parents." >>>> I think I will need two models (one for outcomes rated by the parent, >>>> and >>>> one for outcomes rated by the child). I have measured outcomes with a >>>> parent self report (e.g, Child Depression Inventory parent version) and >>>> an >>>> adolescent self-report version(e.g, Child Depression Inventory child >>>> self-report version). >>>> >>>> >>>> Yes,my primary interest is in Intervention vs Control, specifically in >>>> whether the amount (or pattern) of change varies by Condition on my >>>> outcome variables (e.g. child depression). >>>> >>>> It's a group randomised design, and unfortunately I do have a slight >>>> selection bias (more distressed parents, but not children in the >>>> intervention group). >>>> >>>> I hope this answers your clarification. Does my syntax get at all the >>>> levels? >>>> >>>> Kind regards from Melbourne, >>>> >>>> Christiane >>>> >>> >>> >>> ----- >>> -- >>> Bruce Weaver >>> bweaver@lakeheadu.ca >>> http://sites.google.com/a/lakeheadu.ca/bweaver/ >>> >>> "When all else fails, RTFM." >>> >>> NOTE: My Hotmail account is not monitored regularly. >>> To send me an e-mail, please use the address shown above. >>> >>> -- >>> View this message in context: >>> http://spssx-discussion.1045642.n5.nabble.com/Mixed-Effects-Model-for-group-randomised-trial-tp4447503p4461642.html >>> Sent from the SPSSX Discussion mailing list archive at Nabble.com. >>> >>> ===================== >>> To manage your subscription to SPSSX-L, send a message to >>> LISTSERV@LISTSERV.UGA.EDU (not to SPSSX-L), with no body text except the >>> command. To leave the list, send the command >>> SIGNOFF SPSSX-L >>> For a list of commands to manage subscriptions, send the command >>> INFO REFCARD >>> >> >> ===================== >> To manage your subscription to SPSSX-L, send a message to >> LISTSERV@LISTSERV.UGA.EDU (not to SPSSX-L), with no body text except the >> command. To leave the list, send the command >> SIGNOFF SPSSX-L >> For a list of commands to manage subscriptions, send the command >> INFO REFCARD >> > > > ----- > -- > Bruce Weaver > bweaver@lakeheadu.ca > http://sites.google.com/a/lakeheadu.ca/bweaver/ > > "When all else fails, RTFM." > > NOTE: My Hotmail account is not monitored regularly. > To send me an e-mail, please use the address shown above. > > -- > View this message in context: http://spssx-discussion.1045642.n5.nabble.com/Mixed-Effects-Model-for-group-randomised-trial-tp4447503p4463258.html > Sent from the SPSSX Discussion mailing list archive at Nabble.com. > > ===================== > To manage your subscription to SPSSX-L, send a message to > LISTSERV@LISTSERV.UGA.EDU (not to SPSSX-L), with no body text except the > command. To leave the list, send the command > SIGNOFF SPSSX-L > For a list of commands to manage subscriptions, send the command > INFO REFCARD > ===================== To manage your subscription to SPSSX-L, send a message to LISTSERV@LISTSERV.UGA.EDU (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD ```

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