
Message/Author 

Len Simms posted on Wednesday, September 07, 2011  2:02 pm



I would like to test DIF across two groups using the bifactor model. in particular, I need the ability to test for differences in both loadings and difficulties across the two groups along the general factor. How can this be done with mplus? Also, when mplus does DIF, how does it deal with groups that have different mean levels on the underlying general factor? In a typical IRT analysis, some adjustments are needed on the betas if there is a mean theta difference across groups. How is this done in mplus? Thanks, Len Simms 


You give labels to the parameters of the loadings and thresholds that you are interested in, and make those labels the same vs different in the groups you want to analyze. 2 times the loglikelihood difference for the two models gives the chisquare test of DIF if you use ML. With WLSMV you use DIFFTEST. The different groups have different factor means and variances as the default. No adjustements are needed. 

Len Simms posted on Wednesday, September 07, 2011  9:07 pm



Thank you Dr. Muthen. That should get us started. 


Hi Dear Linda/Bengt, I would like to fit a bifactor IRT model in Mplus. I have come up with the following syntax , items are mixed items format test, Is it correct model has been set Thank you. Sincerely, mohammad ahmadi VARIABLE : NAMES ARE i1i147; USEVARIABLES ARE i2i147; CATEGORICAL ARE i2i147; Missing are all (9) ANALYSIS : ESTIMATOR IS ML; LINK IS LOGIT; MODEL: G by i2* i3i147; G@1; [G@0]; F1 BY i2i14*; F1@1; [F1@0]; . . F10 BY i129i147*; F10@1; [F10@0]; ! uncorrelated factors G with F1F10@0; F1 with F2F10@0; . . F9 with F10@0; ! Item thresholds free ! First threshold [i2$1]; . . [i147$1]; ! Second threshold [i2$1]; [i3$1]; . [i146$1]; [i147$1]; PLOT : TYPE IS PLOT1 PLOT2 PLOT3; SAVEDATA : SAVE = FSCORES; FILE IS bimono.dat; OUTPUT : STDYX; RESIDUAL TECH1 TECH5 TECH8 TECH10; 


This looks correct but note that the thresholds are free by default. 

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