On occasion, generalized linear models for counts based on Poisson or overdispersed count distributions may encounter lack of fit due to disproportionately large frequencies of zeros. Three alternative types of regression models that utilize all the information and explicitly account for excess zeros are examined and given general formulations. A simple mechanism for added zeros is assumed that directly motivates one type of model, here called the addcd-zero type. particular forms of which have been proposed independently by D. LAMBERT (1992) and in unpublished work by the author. An original regression formulation (the zero-altered model) is presented as a rcduad form of the two-part model for count data, which is also discussed. It is suggested that two-part models be used to aid in development of an added-zero model when the latter is thought to be appropriate.

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Recent discovery and characterization of APOAV suggests a role in metabolism of triglyceride (TG)-rich lipoproteins. Previously, variation at the APOAV locus was shown to modestly influence plasma TGs in normolipidemic samples. The aims of this study were to assess the effects of a polymorphism in APOAV (T-1131C) in terms of its frequency among three dyslipidemic populations and a control population, differences of allele frequency across available ethnic groups, and associations with specific lipoprotein TG and cholesterol compartments. We found a striking elevation in the frequency of the rare allele in a Chinese population ( P ‫؍‬ 0.0002) compared with Hispanic and European populations. The rare allele of the polymorphism was associated with elevated plasma TG ( P ‫؍‬ 0.012), VLDL cholesterol ( P ‫؍‬ 0.0007), and VLDL TG ( P ‫؍‬ 0.012), LDL TG ( P ‫؍‬ 0.003), and HDL TG ( P ‫؍‬ 0.016). Linear regression models predict that possession of the rare allele elevates plasma TG by 21 mg/dl ( P ‫؍‬ 0.009) and VLDL cholesterol by 8 mg/dl ( P ‫؍‬ 0.0001), and reduces HDL cholesterol by 2 mg/dl ( P ‫؍‬ 0.017). The association of the polymorphism with altered lipoprotein profiles was observed in combined hyperlipidemia, hypoalphalipoproteinemia, and hyperalphalipoproteinemia, and in controls. These findings indicate that APOAV is an important determinant of plasma TG and lipoprotein cholesterol, and is potentially a risk factor for cardiovascular disease. -Aouizerat, B.