[Show abstract][Hide abstract] ABSTRACT: Students in statistics service courses are frequently exposed to dogmatic approaches for evaluating the role of randomization in statistical designs, and inferential data analysis in experimental, observational and survey studies. In order to provide an overview for understanding the inference process, in this work some key statistical concepts in probabilistic and nonprobabilistic sampling are discussed. The statistical model constituting the basis of statistical inference is postulated and a brief review of the finite population descriptive inference and a quota sampling inferential theory are provided. Some comments on distinct approaches for conducting inferences in probabilistic and nonprobabilistic samples are adduced.
International Journal of Mathematical Education 11/2010; November 01(2002):819-828.
[Show abstract][Hide abstract] ABSTRACT: We propose a mixed-effects linear model for analyzing growth curves data obtained using a two-way classification experiment. The model combines an unconstrained means model and a regression model on the time, in which the coefficients are con-sidered random. The model allows for experimental unit covariates so as to study the trend and the variability of the individual growth curves. Comments on data analysis strategies are provided. An application of the model is illustrated using a data-set comes from a chrysanthemum growth experiment. Resumen Proponemos un modelo lineal de efectos mixtos para analizar datos de curvas de crecimiento de un experimento con dos criterios de clasificación. El modelo combina un modelo no restringido de medias y un modelo de regresión sobre el tiempo, en el cual los coeficientes son considerados aleatorios. El modelo considera covariables a nivel de la unidad experimental para estudiar la tendendia y la variabilidad de las curvas de crecimiento. Se proporcionan comentarios sobre estrategias de analisis de datos. Se ilustra la aplicación del modelo usando un conjunto de datos de un experimento de crecimiento de crisantemos. Palabras clave: Modelos de regresión lineal multinivel, modelos de coeficientes aleatorios, modelos de medias, estrategias de analisis de datos.
Revista de Matemática: Teoría y Aplicaciones. 08/2009; 11(2).
[Show abstract][Hide abstract] ABSTRACT: In this paper we review the main proposed approximations to percentiles of the noncentral t-distribution. The approximations are examined for their acuracy over a wide range of values of the parameters of the distribution and for several percentil values. Tables summarizing the approximations are included.
Revista de Matemática: Teoría y Aplicaciones. 02/2009; 10(1-2).
[Show abstract][Hide abstract] ABSTRACT: In this work simultaneous confidence intervals for the variance components in the two-way balanced crossed random effects model with interaction have been derived under the usual assumptions of normality and independence of random effects. The intervals are conservative in the sense that the true confidence coefficient is as large as preassigned value. The formulas are illustrated using published data with SAS outputs.
[Show abstract][Hide abstract] ABSTRACT: It is known that vitamin C status is compromised in smokers. The vitamin C status of nonsmokers who are exposed to environmental tobacco smoke (ETS) is now being elucidated.
We assessed vitamin C status in children who were either exposed or not exposed to ETS, and we sought to associate changes in vitamin C status with the amount of ETS exposure.
The study group included 512 children aged 2-12 y; 50% of them were exposed to ETS in the home because their parents smoked. Dietary intake of vitamin C, obtained with a 24-h recall questionnaire, and blood ascorbate concentrations were compared in the exposed and unexposed groups. Smoke exposure was assessed by measuring a biomarker, urinary cotinine. Age, sex, and body mass index were examined as potential correlates of vitamin C status in each exposure category.
Plasma ascorbate concentrations were lower, by 3.2 micro mol/L on average, in ETS-exposed children than in unexposed children who consumed equivalent amounts of vitamin C; this was a highly significant difference (P = 0.002). This reduction in plasma ascorbate occurred even with very low exposure to ETS.
ETS can reduce concentrations of ascorbate, an important blood antioxidant, even when the amount of smoke exposure is minimal. Children exposed to ETS should be encouraged to consume increased amounts of foods rich in vitamin C or should be given the equivalent amount of this vitamin as a supplement.
American Journal of Clinical Nutrition 02/2003; 77(1):167-72. · 6.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The use of a project-based approach for designing a one-year graduate level programme in applied statistics is addressed. The pedagogical approach, academic setting, and learning activities are described in a multidisciplinary context. Some comments on implementation of such a programme based on the results from five successive graduating classes at the University of Veracruz (1994–2000) are included.
International Journal of Mathematical Education 01/2003; 34(1):57-63.
[Show abstract][Hide abstract] ABSTRACT: This study was designed to determine among various personal, socioeconomic, and environmental factors those which had the greatest influence on exposure to environmental tobacco smoke (ETS) in a population of children residing in a tropical environment and to compare these results with those obtained in the literature of tobacco exposed children in temperate climates. Urine specimens were collected from 606 healthy Puerto Rican children (2-12 years) living in an industrial area and analyzed for cotinine, a quantitative biomarker for exposure to ETS. Parents completed a questionnaire covering smoking habits and socioeconomic information. Seventy per cent of the children were reported to be exposed to ETS, 50% resulting from exposure to smoke from either or both parents. Major determinants to ETS exposure were found to be presence of smoker, number of smokers, identity of smoker, number of cigarettes smoked in the household and child age with the youngest children suffering twice the exposure of older children. Non-determinants were exposure to smoke other than from the parent, sex of the child, season of the year and several socioeconomic factors including civil and employment status of the mother, mother's age and educational background and whether food stamps were being received. Results of a multiple regression analysis showed that our predictors accounted for 40% of cotinine appearing in the urine. Reasons for this relatively low value may be due in part to precision of our analytic method and lower levels of ambient smoke in our population vs. others that reported higher R(2) values. Predictions from questionnaire information for high ETS exposure were not always the same as those indicated by urinary cotinine emphasizing that the bioindicator, which indicates the actual inhalation of ETS, is a better predictor of exposure than responses from a questionnaire.
Nicotine & Tobacco Research 03/2001; 3(1):61-70. · 2.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The effects of nonnormality on confidence intervals for the between group variance component, where samples for both the random effects are taken from an Edgeworth population, are investigated using two classical approximate methods based on χ 2 and F distributions. It is found that the intervals are much more affected by the kurtosis of group effects than by the kurtosis of the error effects. Moreover, the intervals based on the F approximation are shorter than those based on the χ 2 approximation.
Journal of Applied Statistical Science 01/1999; 8(2).
[Show abstract][Hide abstract] ABSTRACT: The problems and challenges encountered in teaching biostatistics to medical students and professionals are considered. Some suggestions and tips which may help to overcome some of these problems and enhance certain aspects of biostatistics teaching and learning are presented.
International Journal of Mathematical Education 01/1999; 30(2).
[Show abstract][Hide abstract] ABSTRACT: Many studies of environmental tobacco smoke (ETS) have been conducted in northern, industrialized countries. As yet, however, no studies have been carried out on ETS exposure with nonsmokers living in tropical environments.
Urine specimens were collected from 175 healthy Puerto Rican children (2-11 years) living in an industrial area and were analyzed for cotinine, a quantitative biomarker for exposure to ETS. Their parents completed a questionnaire covering smoking habits.
Seventy percent of children were exposed to ETS. Quantitatively, exposure to smoke in households consuming more than 1 pack per day (ppd) caused a doubling of cotinine excretion compared with households consuming less than 1 ppd. Smoke from mothers made the greatest contribution to cotinine, followed by smoke from fathers, with smoke from other persons having no effect. Degree of exposure was inversely related to age of the child.
Young children (2-4 years) were detected to have significantly greater exposure to ETS than older children (5-11 years) and in the younger group the effect seemed to be from the mother's smoking much more than the father's, with other persons contributing negligible amounts. This suggests an obvious strategy for prevention of exposure to ETS in young children.
Preventive Medicine 01/1997; 26(1):1-7. · 3.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper is a compendium of exact and asymptotic formulae and tables for estimating the sample size in a clinical trial with two treatment groups and a dichotomous outcome. The paper provides separate formulae for equal and unequal treatment group sizes, formulae for the calculation of power given the sample size, and complete references for all formulae and tables cited.
Statistics in Medicine 02/1996; 15(1):1-21. · 2.04 Impact Factor