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Date:   Tue, 1 Dec 2009 08:02:56 -0800
Reply-To:   Oliver Kuss <Oliver.Kuss@MEDIZIN.UNI-HALLE.DE>
Sender:   "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:   Oliver Kuss <Oliver.Kuss@MEDIZIN.UNI-HALLE.DE>
Organization:   http://groups.google.com
Subject:   Re: Latent Class Analysis - Question
Comments:   To: sas-l@uga.edu
Content-Type:   text/plain; charset=ISO-8859-1

On 1 Dez., 16:23, Ryan <ryan.andrew.bl...@gmail.com> wrote: > On Dec 1, 9:15 am, Oliver Kuss <Oliver.K...@medizin.uni-halle.de> > wrote: > > > > > > > On 1 Dez., 11:43, Ryan <ryan.andrew.bl...@gmail.com> wrote: > > > > On Dec 1, 2:48 am, Oliver Kuss <Oliver.K...@medizin.uni-halle.de> > > > wrote: > > > > > On 1 Dez., 02:16, Ryan <ryan.andrew.bl...@gmail.com> wrote: > > > > > > Hi, > > > > > > Let me apologize in advance for asking the same question twice. I > > > > > figured I'd give it another shot. > > > > > > Has anyone seen/developed code to run a random effects latent class > > > > > analysis in SAS. Let's say we have three dichotomous indicator > > > > > variables (0=No, 1=Yes) that we hypothesize load on a latent class > > > > > variable (with 3 classes). > > > > > > A simple example I just made up: We suspect that there are three > > > > > classes of people who use illicit substances (class 1 = non-users/ > > > > > abstainers, class 2 = casual users, class 3 = addicts). Assume we > > > > > cannot measure directly if someone belongs to any of these classes, > > > > > but we have 3 indicator variables as indicated previously. Let's also > > > > > assume that we have two cases per person (measured at equal > > > > > intervals)... > > > > > > /----------------------------------------------/ > > > > > Person Time X1 X2 X3 > > > > > 1 1 0 1 1 > > > > > 1 2 1 0 1 > > > > > 2 1 0 0 0 > > > > > 2 2 0 0 1 > > > > > . > > > > > . > > > > > > N > > > > > /----------------------------------------------/ > > > > > > Does anyone know how to construct code (presumably in nlmixed) to run > > > > > a random intercept LCA and compute the following: > > > > > > (1) Probability that a positive response on each item is associated > > > > > with a particular class > > > > > (2) Probability that each case is associated with a particular class > > > > > (3) Any indication that the number of classes we selected does not > > > > > yield the best fitting model. I assume re-running the model assuming 2 > > > > > classes, 4 classes, etc. and comparing AICs/BICs might work. > > > > > > Any thoughts/recommendations/references would be great. > > > > > > Thanks, > > > > > > Ryan > > > > > Dear Ryan, > > > > it seems that you also have a longitudinal structure in your data set > > > > with two (or even more) observations for each person. Then you should > > > > definitely look at PROC TRAJ (http://www.andrew.cmu.edu/user/bjones/), > > > > a user-written SAS prodecure that fits discrete mixture models to > > > > longitudinal data. I once worked with it and it did fine. Before final > > > > publication of the results I also coded the model with PROC NLP and it > > > > yielded the same results. So you might also use PROC NLP or PROC > > > > NLMIXED for latent class models. > > > > > Hope that helps, > > > > Oliver- Hide quoted text - > > > > > - Show quoted text - > > > > Thanks for responding, Oliver. Thank you for the info about TRAJ > > > procedure. I would prefer to run the model using the NLMIXED > > > procedure. I assume it is possible to run such a model as evidenced by > > > a post by Dale a while back: > > > >http://www.listserv.uga.edu/cgi-bin/wa?A2=ind0503a&L=sas-l&D=0&P=26375 > > > > What's confusing to me about Dale's post is the dependent variable. > > > What exactly would be the dependent variable in an LCA such as the > > > example I made up? > > > > Ryan- Zitierten Text ausblenden - > > > > - Zitierten Text anzeigen - > > > Dear Ryan, > > I got your point. I admittedly do not know how such a model can be > > coded with PROC NLMIXED but I have two more hints which might be > > useful: > > 1. There is a SUGI paper using PROC CATMOD (http://www2.sas.com/ > > proceedings/sugi31/201-31.pdf) for LCA and 2. There is a user-written > > SAS procedure LCA (http://methodology.psu.edu/index.php/downloads/ > > proclcalta) whose first example has four binary indicators which > > should be grouped in two classes (similar to your problem, without a > > "response"). Maybe you can use PROC LCA for achieving the results for > > your data set and then use the description of the model in the PROC > > LCA handbook for translating the model into PROC NLMIXED. > > > Yours, > > Oliver- Hide quoted text - > > > - Show quoted text - > > Who knew there were so many SAS procedures that could handle an LCA > model?! I will certainly try to run it (without the REs) using one of > these other procedures. Regardless, however, if I want to run a random > effects LCA in SAS, I'm probably going to have to figure out how to do > it in nlmixed. Thanks again for the info. -Ryan- Zitierten Text ausblenden - > > - Zitierten Text anzeigen -

Dear Ryan, I'm in a mood today for searching for LCA software ;-))) Give the LCR macro of Diana L. Miglioretti a try (http:// www.grouphealthresearch.org/perpages/migliore/downlds/LCREG2.SAS) It makes explizit the IML code for fitting the models and maybe is an even better starting point for coding the model in NLMIXED.

Good luck, Oliver


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