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June Zhou posted on Tuesday, January 29, 2013  8:57 pm



I am trying to run multilevel IRT Rasch models in Mplus and I have some questions listing as below: 1. In the 2level 1PL IRT model, items (level1) were nested in students (level2), can I use Ex.7.27 and constrain all of loadings to be 1? How to add studentlevel predictors in the model? 2. In the 3level 1PL IRT model, items (level1) were nested in students (level2) and students were nested in schools (level3), should I use EX.10.5 and constrain all of loadings to be 1? How to add schoollevel predictors in the model? Thank you in advance. 


1. This is not a multilevel model in Mplus. See Example 5.2 and 5.5. 2. This is a twolevel model in Mplus. See Example 9.7 which includes a betweenlevel covariate. 

June Zhou posted on Wednesday, January 30, 2013  12:03 pm



Thank you very much for your advice, Dr. Muthen. I really appreciate your help. 

June Zhou posted on Monday, April 01, 2013  4:15 pm



I am trying to run an MCMC Rasch model in Mplus. I have two questions: 1. When I tried to constrain item discrimination parameters (factor loading) to be 1, I found I only got identical loadings but they were not 1. Why? 2. In MCEX5.5, is the code "[u1$1u10$1*.5 u11$1u20$1*.5]" meant to fix item difficulty parameters of the first 10 items to be .5 and the last 10 items .5? If yes, can I change the values to 2.5, 0.5, 0, 0.5, and 2.5 since the range of item difficulty parameter is (3, 3)? Thank you in advance! 


1. If you hold them equal, one parameter will be estimated. That won't happen if you fix them @1. 2. Yes. The range of difficulty parameters depend on the variance of the factor. Using 3 implies variance one. 

June Zhou posted on Wednesday, April 03, 2013  3:36 pm



Thank you so much for your prompt reply, Dr. Muthen! I really appreciate your help. This time I am going to do a Monte Carlo study for a 2level Rasch model with within and betweenlevel variables in Mplus. Below are my codes. Compared with mcex10.5. I am not sure whether I defined my thresholds (item difficulty) accurately or not. Would you please give me some advice? Thank you for your time in advance. montecarlo: names = u1u5 x1 x2; generate = u1u5(1); categorical = u1u5; nobs = 200; ncsizes = 2; csizes = 6 (20) 2 (40); seed = 48459; nreps = 500; repsave=all; save=con1*.dat; within = x1; between = x2; ANALYSIS: TYPE IS TWOLEVEL; model population: %Within% [x1*0]; x1@1; fw by u1@1 u2u5*1; fw*1; fw ON x1*.25; %Between% [x2*0]; x2@1; fb by u1@1 u2u5*1; fb*.1111; fb ON x2*.25; [u1$1*2.5 u2$1*1.5 u3$1*0 u4$1*1.5 u5$1*2.5]; 


Looks ok. But you want more clusters than 8 in a twolevel analysis. And you want to specify MODEL as well, not only MODEL POPULATION. 

June Zhou posted on Thursday, April 04, 2013  2:00 pm



Thank you very much for your advice, Dr. Muthen. Could you please tell me the reason why the thresholds of variables on the between and within levels exist only on the between level. In other words, why I couldn't define thresholds in within level? Thanks! 


Each item has just one set of thresholds (one if binary, several if polytomous) and the convention is that they are reported on the Between level. They cannot also be identified on the within level. This is in line with means/intercepts in regular multilevel analysis. 

June Zhou posted on Monday, April 08, 2013  8:00 am



I see. Thank you so much for your reply! Sorry, I have another question: is there a statistic estimating the latent person ability parameter in IRT model in Mplus? Thank you in advance! 


You request factor scores using FSCORES. 

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