I have 4 waves of measurement on childrenís behavioral problem scores (normally distributed) and 4 corresponding measurements on the # of placements experienced between waves for 5500 children in child welfare system. The placement data is extremely skewed and has many zeros. The %s of children experiencing, respectively, 0, 1, 2, and 3 or more placements in the four waves are: wave 1: 85, 10, 3, 1; wave 2: 75, 12, 7, 6; wave 3: 88, 7, 3, 2 and wave 4: 95, 2, 1, 2.
I am using a cross-lagged panel design, with the # of placements predicting behavior problems and visa versa. For now, I am treating # of placements as a 4-category ordinal variable, but I wonder whether the % of zeros and/or skew are such that this model ceases to be viable. How do I assess?
Would a zero-inflated poisson model be preferred? A problem is that the inflated portion canít be used as a predictor (page 521). Can mplus build overdispersion into a poisson model?
The semi-continuous model seems to hold possibilities. If I donít switch to a growth model, can the binary and continuous variables created from the placement variable at an earlier wave function as predictors of outcomes in a subsequent one?
I may exclude wave 4, due to its low variability and other problems. I can exclude a subgroup of 1000 kids who experienced hardly any placements. Would these changes help?
In Version 5.1, we added several models for count variables. These are documented in the Version 5.1 Language Addendum and the Version 5.1 Examples Addendum which are on the website with the user's guide. Try the negative binomial model.
I would not use a semicontinuous model. There are not enough values in the tail.
See how the negative binomial model works before you consider other changes.