Significance of variances of growth f... PreviousNext
Mplus Discussion > Latent Variable Mixture Modeling >
 Liesbeth Bruckers posted on Thursday, April 19, 2007 - 3:58 am

In growht mixture models: can you use the t statistic to evaluate the significance of the variance of the latent slope ? I have encountered this in a couple of articles. Should one not use a mixture of chisquare distributions to test for this ?

Thanks for your advice,
 Bengt O. Muthen posted on Thursday, April 19, 2007 - 8:41 am
A mixture of chi-squares is the proper thing to use in principle. Many in the field seem to use the z scores as an approximation (which may not be good enough).
 Liesbeth Bruckers posted on Monday, April 23, 2007 - 6:14 am
The test statistic is calculated as a difference in twice the log likelihood not ? Or can one use the square of the z-statistic ?

 Linda K. Muthen posted on Monday, April 23, 2007 - 8:01 am
Z squared is equal to two times the loglikelihood difference. Neither of them is a mixture of chi-squares. Both of them are used in practice.
 Liesbeth Bruckers posted on Wednesday, April 25, 2007 - 4:18 am
Dear Linda,

I was wondering. Suppose I have a GMM, with two classes. For both classes I specify a random intercept and slope. To test the significance of the variance of the slope growth factor I fit a model including a random slope and a model with a fixed slope. I compare the log likelihood values of these two models. Is this allowed ? Are this still nested models; what if the partitioning of the patients in two classes is different in these two models ?

Thanks for your reaction,

 Linda K. Muthen posted on Wednesday, April 25, 2007 - 8:22 am
Neither the z-test nor the LL difference test would be appropriate here although they are probably used in practice as an approximation.
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