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?
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 chi-square 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
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 i1-i147; USEVARIABLES ARE i2-i147; CATEGORICAL ARE i2-i147; Missing are all (9) ANALYSIS : ESTIMATOR IS ML; LINK IS LOGIT; MODEL: G by i2* i3-i147; G@1; [G@0]; F1 BY i2-i14*; F1@1; [F1@0]; . . F10 BY i129-i147*; F10@1; [F10@0]; ! uncorrelated factors G with F1-F10@0; F1 with F2-F10@0; . . F9 with F10@0;
Hi Dear Linda/Bengt I would like to fit a Bifactor Multidimensional Item Response Theory (MIRT) Differential Item Functioning (DIF) Model With a Bayesian Estimation in Mplus. But I want to do with An iterative item purification procedure and In addition, the set of anchor items contain at least one item from each testlet that free of DIF to ensure that the anchor set does not contain items that show DIF. 1-Is there such a facility in this software? 2-can compute impact(difference between latent ability of groups? 3- can compute testlet effect? 4-can compute variance of testlet effect? 5- can compute mean of ability distribution? Thank you. Sincerely, mohammad ahmadi