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From |
"Verkuilen, Jay" <JVerkuilen@gc.cuny.edu> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: suggested references about the variables to include in zero-inflated portion of zinb? |

Date |
Mon, 27 Oct 2008 14:28:19 -0400 |

Steven Samuels wrote: >>I'd like to amend one point in my previous post-that too many zero values are cause for suspicion. I spoke to a friend who is familiar with diagnostic psychological scales. She pointed out that the fewer the items in a scale, the more likely zeros would be. In her experience, mean scores were often small and lumps at zero were common. I've found the same an eight binary item brief screen for mental illness--most people had no "positive" items.<< This is indeed a common problem but one for which I think the zero inflation models are quite sensible contenders. The ZIP model is a latent class model with latent class 1 being a degenerate Poisson and latent class 2 being a non-degenerate Poisson, i.e., people with genuinely no symptoms and people who reported no symptoms but . While it's not a model available in common programs, a Zero Inflated Binomial would be similar. The problem of a diagnostic scale showing no symptoms for many respondents and then some for others makes sense as a mixture problem (potentially---it depends on the symptoms and how you think about them). As it is common and sensible to use covariates to predict latent class membership, I think for a lot of these kinds of diagnostic scale problems, having covariates predict zero response would be very useful. These covariates are predicting, in essence, the presence of symptoms at all while the covariates for the scale (possibly distinct, possibly overlapping) predict which latent class a subject was in. JV * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**Re: st: suggested references about the variables to include in zero-inflated portion of zinb?***From:*Maarten buis <maartenbuis@yahoo.co.uk>

**Re: st: suggested references about the variables to include in zero-inflated portion of zinb?***From:*Steven Samuels <sjhsamuels@earthlink.net>

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