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Dear Linda and Bengt, I am trying to run a zeroinflated 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. 


1) The way you are doing it is correct. You might want to first try a noninflated Poisson model. If you have problems, send your materials and license number to support@statmodel.com. 2)ex 61.6 is a twopart model. This is a little different than ZIP. ZIP is a 2class model where there are 2 classes that can produce a zero value, while twopart model is a singleclass model. With twopart you would have to treat the positive number of cigarettes as continuouslognormal; 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. 

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