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Ali posted on Monday, February 17, 2014 - 9:20 pm
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Hello I am using Mixture IRT model to fit 1 calss, 2 class, and 3 class with 20 data set. In 2 class, there is no any problem to fit, also, THE BEST LOGLIKELIHOOD VALUE HAS BEEN REPLICATED. But , in 3 class , sometimes it showed THE BEST LOGLIKELIHOOD VALUE HAS BEEN REPLICATED , but meanwhile shows :THE STANDARD ERRORS OF THE MODEL PARAMETER ESTIMATES MAY NOT BE TRUSTWORTHY FOR SOME PARAMETERS DUE TO A NON-POSITIVE DEFINITE FIRST-ORDER DERIVATIVE PRODUCT MATRIX.; and :ONE OR MORE PARAMETERS WERE FIXED TO AVOID SINGULARITY OF THE INFORMATION MATRIX.; Are results trustful such item parameters and the values of BIC? Or does it mean that model can・t fit the data? In addition, sometimes the results of 3 classes showed :THE BEST LOGLIKELIHOOD VALUE WAS NOT REPLICATED. : and I increased the starting value = 100 20 to 1000 200 , but it still showed that :THE BEST LOGLIKELIHOOD VALUE WAS NOT REPLICATED. : Does that mean the model can・t fit the data too? Or should I increase starting values more? |
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The best loglikelihood must be replicated. Even when it is replicated, there can be other problems. I would need to see the output with the error message and your license number at support@statmodel.com to determine what the problem is. |
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Ali posted on Monday, February 24, 2014 - 7:45 am
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Thanks! Can I obtain the estimate of individual ability while I am using mixture IRT model? Or, I just can only get the membership and mean of group ability ? |
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You get the individual ability estimate for each latent class and also the ability estimate mixed over classes. For a factor names F, they are called F_C and F, respectively. |
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Ali posted on Monday, February 24, 2014 - 4:49 pm
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So, do you mean the output which I can get the the estimated of ability for each class and the whole sample's estimated ability ? But, could I get the estimated of ability of each person in a class? For example, if there are 200 people in class 1 , could I also get 200 people's estimated ability in class 1? |
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I am talking about each person's factor score - you get 2 versions of each person's factor score (class-specific and mixed over classes). You get that using the Savedata command and Save - fscores. |
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Ali posted on Monday, March 03, 2014 - 7:04 pm
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Thank you. I am trying to find the fitted model in a mixture Rasch model. I fitted the data in 2 classes , 3 classes and 4 classes. BIC for 2,3,4 classes are 45190.796,45053.329 45083.17.From the BIC value, 3 classes should fit well, but the output gave me the message"ONE OR MORE PARAMETERS WERE FIXED TO AVOID SINGULARITY ...". Should I choose the 3- classes model? |
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It depends which parameter was fixed and why. Please send the output and your license number to support@statmodel.com. |
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