Growth model using zero-inflated pois... PreviousNext
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 Jim Prisciandaro posted on Thursday, June 28, 2007 - 3:27 pm
Dear Linda and Bengt,

I am trying to run a zero-inflated poisson model for a count variable (cigarette use), using 35 points of assessment. I attempted to estimate a model using example 6.7 in the manual, however the model failed to estimate.

My questions are the following:
1) Since I am only estimating an intercept model (no slope), is the example 6.7 applicable for me? I deleted "s" and "si" as well as "s@0" and "si@0" in the Model statement. Is there anything else I need to adjust if I want to estimate an intercept only model?

2) I was also considering estimating this model using example 6.16 of the manual. However, it appears very similar to the model in the example 6.7. What are the differences between these two models? And which one would you recommend I use for my case (i.e., an intercept only model for a cigarette use variable, assessed weekly over a course of a semester; 35 points of assessment)?

Thank you very much for your help.
 Bengt O. Muthen posted on Thursday, June 28, 2007 - 4:34 pm
1) The way you are doing it is correct. You might want to first try a non-inflated Poisson model. If you have problems, send your materials and license number to support@statmodel.com.

2)ex 61.6 is a two-part model. This is a little different than ZIP. ZIP is a 2-class model where there are 2 classes that can produce a zero value, while two-part model is a single-class model. With two-part you would have to treat the positive number of cigarettes as continuous-lognormal; the count option should not be used because Mplus does not provide a truncated (at zero) Poisson. If you have most people at zero, you may want to use ZIP.
 Qiana Brown posted on Wednesday, June 25, 2014 - 7:04 am
Hello,

1. Can I use the growth model for parallel processes if my outcomes are not continuos?

I have one outcome (past month smoking) that is binary, and another outcome that is continuous. I would like to model them using the parallel process growth model, but
section 6.13 in the User's guide only mentions the parallel process for continuous outcomes.

2. My binary smoking variable has several zeros at each time point. About 82% of the participants are zeros (did not smoke in the past month). Should I used a zero-inflated poisson growth model in this case, or will a growth model for binary outcomes suffice? Also, can the zero-inflated poisson growth model be modeled in the parallel processes framework?

Thank you
 Bengt O. Muthen posted on Wednesday, June 25, 2014 - 5:53 pm
1. Yes. A binary and a cont's process can be handled.

2. You don't want to use Poisson if you have only 2 response categories. Only if you smoking variable really consists of counts. Yes, you can combine a zIP model for one of the parallel processes with a cont's variable process.
 Qiana Brown posted on Sunday, July 06, 2014 - 10:36 am
Hello,
I used a parallel processes growth model with a MLR estimator to model a binary and continuous outcome. Should the estimates be interpreted as if both outcomes were continuous? Also, are there any specific model fit indices that I should consider in this case? Would you please recommend a paper that might help me?

Thank you
 Bengt O. Muthen posted on Sunday, July 06, 2014 - 11:26 am
The analysis and interpretation of binary growth models is a large topic that we cover in Topic 3 of our short courses on our website. See the video and handout, slides 185-212. This also gives references.

See also the paper on our website under Papers, Growth Modeling:

Masyn, K., Petras, H. and Liu, W. (2013). Growth Curve Models with Categorical Outcomes. In Encyclopedia of Criminology and Criminal Justice (pp. 2013-2025). Springer.
 Qiana Brown posted on Sunday, July 06, 2014 - 1:03 pm
Thank you. I appreciate the resources.
 Qiana Brown posted on Wednesday, July 09, 2014 - 11:04 am
Hello,
The slides and lecture for Topic 3 were helpful. I still have a question regarding the interpretation of the estimates from my parallel processes growth model.

In my model, one outcome is binary and the other continuous. The estimator is MLR and the link is logit - so are the slopes log odd ratios and the thresholds negative log odds?

Thank you
 Bengt O. Muthen posted on Wednesday, July 09, 2014 - 4:02 pm
Right.
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