The degrees of freedom is the difference in the number of parameters. And, yes, 3.84 is the chi-square cutoff for one degree of freedom.
You should subtract the chi-square of the least restrictive model from the chi-square of the most restrictive model. This should be positive.
newuser posted on Wednesday, January 06, 2010 - 7:09 pm
Hi, Dr. Muthen; I am trying to compare an interaction model to one without the interaction. I am wondering 1. Can I just take the ¨C2*difference between loglikelihood to get the chi-square, or should I do the TRd? From the earlier postings, it seems ¨C2*LL can be used, then when should the TRd be used? 2. Is the difference between the degree of freedom from the two models¡¯ ¡°Number of Free Parameters¡± under ¡°info criteria¡±? 3. Only the loglikelihood of the two ¡°H0¡± should be compared, not the ¡°H1¡±? 4. If the two models can be expressed as Y on X, W, W*X is compared to Y on X, W. I have trouble understanding why introducing the interaction term to the Y on X, W model make the two models nested, because after the interaction is introduced, the models are not identical anymore. 5. the output for the main-effect-only- model don't have a Scaling correction factor reported, is there a way to request one? Thanks for your help in advance! I tried to post a similar message earlier, but don¡¯t see it, so trying it again.
The difference in degrees of freedom is the difference in the number of parameters. The test is -2 times the H0 loglikelihood difference.
You should obtain a scaling correction factor if you are using a recent version of Mplus and the estimator requires one. Please send the output and your license number if you think there should be a scaling correction factor and there is not one.
newuser posted on Thursday, January 07, 2010 - 9:34 am
Dear Dr. Muthen, thank you so much for your reply, just to make sure, in getting the "-2 times the H0 loglikelihood difference"you mentioned, do I need to divide that # by the cd (obtained through scaling correction factor and df) to get a TRd? Or do I just use the raw difference from the two likilihoods? Thanks gain!!