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This service applies to problems in using the software and does not include statistical consultation.
Documentation
FAQS
Installation and set-up
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In our experience assisting users, the most often seen cause is that one or more items do not have observations in specified response categories. For example, flexMIRT® flexMIRT™ is told that an item has 4 categories and should fit the Graded(4) model but, in the data, there are not observations that have a value of 2. It is impossible for flexMIRT™ to estimate an intercept between two categories if one of the categories is not seen in the data. Collapsing/recoding the data so observed response values are sequential and specifying an item model with an appropriate number of categories is one way to resolve the problem.
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Some models are very complex and will require longer run times. If you are working with multidimensional models, you can try increasing the number of processors available to flexMIRT™ (default is 1) or decreasing the number of quadrature points provided that the reduced precision of numerical integration does not substantially degrade the estimation quality and stability (default is 49 points over the range -6 to 6). These things can be accomplished, respectively, by using statements such as Processors = 3; and Quadrature = 21, 5.0; for example, in the <Options> section of your flexMIRT™ syntax.
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If providing guessing values for the 3PL, the values in the -prm will need to be logit-guessing, not typical g-parameter values. This PDF provides additional details on the parameterization of the 3PL (and GPC) in flexMIRT™.
Due to several questions regarding conversion of the GPC model, this zip file contains details regarding the Nominal model that flexMIRT™ uses, a labeled example of a -prm file for GPC items, and an example of the conversions needed to go from more typical GPC parameters to the parameters flexMIRT™ expects in the -prm file for scoring.
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Currently there is not an option to save the original parameterization values into the -PRM file, though they are printed in the output. Using the alpha and gamma values saved into the -PRM file and following the formulae in Thissen, Cai, and Bock (2010), specifically the T matrix described in Eq. 3.35 and 3.36 and parameter conversions found in Eqs 3.37 – 3.41, users may obtain the estimates in the original parameterization. Please note that there is an error in Eq. 3.41 in the chapter, which should read:
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I fit a multidimensional model and
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flexMIRT™ is only printing the intercept values (c parameters). I would like thresholds (bs) instead. Can I make flexMIRT™ print those?
As a logistic multidimensional model, threshold parameters are not straight forwardly derived and tend to have far less logical and theoretical basis; for these reasons, flexMIRT™ will not print them. However, you can convert the logistic intercepts into approximate Normal metric threshold parameters using the following method:
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