Guido del Pino

Genetic epidemiology of cholesterol cholelithiasis among Chilean Hispanics, Amerindians, and Maoris

BACKGROUND & AIMS The etiology of cholesterol gallstones is multifactorial, with interactions of genes and the environment. The hypothesis that aborigine cholesterol lithogenic genes are widely spread among Chileans, a population with a high prevalence of gallstones, was tested. METHODS Medical history and anthropometric measurements were obtained and abdominal ultrasonography was performed in 182 Mapuche Indians, 225 Maoris of Easter Island, and 1584 Hispanics. Blood groups, DNA, lipids, and glucose were analyzed. The Amerindian Admixture Index and mitochondrial DNA (mtDNA) assessed the ethnicity and degree of racial admixture. RESULTS Amerindian Admixture Index was 0.8 in Mapuches and 0.4 in Hispanics. All Mapuches, 88% of Hispanics, but none of Maoris had Amerindian mtDNA haplotypes. Age- and sex-adjusted global prevalence of gallstone disease was higher in Mapuches (35%) than in Hispanics (27%) and Maoris (21%). Compared with Hispanics, the youngest group of Mapuches had the greatest corrected risk of gallstones: odds ratios of 6.0 in women and 2.3 in men. In contrast, the gallstone risk in Maoris was lower compared with Hispanics: odds ratios of 0.6 for women and 0.5 for men. CONCLUSIONS Cholesterol lithogenic genes appear widely spread among Chilean Indians and Hispanics. They could determine the early formation of gallstones and explain the high prevalence of gallbladder diseases among some South American populations.

IRT Models for Ability-Based Guessing:

An ability-based guessing model is formulated and applied to several data sets regarding educational tests in language and in mathematics. The formulation of the model is such that the probability of a correct guess does not only depend on the item but also on the ability of the individual, weighted with a general discrimination parameter. By so doing, the possibility that the individual uses his or her ability to some extent for differentiating among responses while guessing is also considered. Some important properties of the model are described and compared with analogous properties of related models. After simulation studies, the model is applied to different data sets of the Chilean Sistema de Medición de la Calidad ń (SIMCE) tests of mathematics and language. The conclusion of this analysis seems relevant—namely, that the examinees use their ability to guess in the language test but not in the mathematics test.

Monte Carlo EM with importance reweighting and its applications in random effects models

In this paper we propose a new Monte Carlo EM algorithm to compute maximum likelihood estimates in the context of random effects models. The algorithm involves the construction of efficient sampling distributions for the Monte Carlo implementation of the E-step, together with a reweighting procedure that allows repeatedly using a same sample of random effects. In addition, we explore the use of stochastic approximations to speed up convergence once stability has been reached. Our algorithm is compared with that of McCulloch (1997). Extensions to more general problems are discussed.

A coordinate free approach to score tests

This paper presents a coordinate free approach to score tests and Cα tests. This approach allows the tests to be understood by analogy to the usual linear model. The invariance propertieswith respect to changes in parametrizations are stressed. Finally, aresult of Pregibon relating score tests to the Pearson statistic is extended.