Modeling hierarchical relationships in epidemiological studies: a Bayesian networks approach

Hierarchical relationships between risk factors are seldom taken into account in epidemiological studies though some authors stressed the importance of doing so, and proposed a conceptual framework in which each level of the hierarchy is modeled separately. The objective of this paper was to implement a simple version of their framework, and to propose an alternative procedure based on a Bayesian Network (BN). These approaches were illustrated in modeling the risk of diarrhea infection for 2740 children aged 0 to 59 months in Cameroon. The authors implemented a (naïve) logistic regression, a step-level logistic regression and also a BN. While the first approach is inadequate, the two others approaches both account for the hierarchical structure but to different estimates and interpretations. BN implementation showed that a child in a family in the poorest group has respectively 89%, 40% and 18% probabilities of having poor sanitation, being malnourished and having diarrhea. An advantage of the latter approach is that it enables one to determine the probability that a risk factor (and/or the outcome) is in a given state, given the states of the others. Although the BN considered here is very simple, the method can deal with more complicated models.Bayesian networks; hierarchical model; diarrhea infection; disease determinants; logistic regression

Multidimensional Nature of Undernutrition: A Statistical Approach

The statistical assessment of undernutrition is usually restricted to a pairwise analysis of anthropometric indicators. The main objective of this study was to model the associations between underweight, stunting and wasting and to check whether multidimensionality of undernutrition can be justified from a purely statistical point of view. 3742 children aged 0 to 59 months were enrolled in a cross-sectional household survey (2004 Cameroon Demographic and Health Surveys (DHS)). The saturated loglinear model and the multiple correspondence analysis (MCA) showed no interaction and a highly significant association between underweight and stunting (P=0), underweight and wasting (P=0); but not between stunting and wasting (P=0.430). Cronbach's alpha coefficient between weight-for-age, height-for-age and weight-for-height was 0.62 (95% CI 0.59, 0.64). Thus, the study of these associations is not straightforward as it would appear in a first instance. The lack of three-factor interaction and the value of the Cronbach's alpha coefficient indicate that undernutrition is indeed (statistically) multidimensional. The three indicators are not statistically redundant; thus for the case of Cameroon the choice of a particular anthropometric indicator should depend on the goal of the policy maker, as it comes out of this study that no single indicator is to be used for all situations.Stunting; Wasting; Underweight; anthropometric measures; Z-score; Loglinear models

Using weight-for-age for predicting wasted children

Background: The equipments for taking body weights (scales) are more frequent in Cameroon health centres than measuring boards for heights. Even when the later exist there are some difficulties inherent in their qualities; thus the height measurement is not always available or accurate. Objective: To construct statistical models for predicting wasting from weight-for-age. Methods: 3742 children a ged 0 to 59 months were enrolled in a cross-sectional household survey (2004 Cameroon Demographic and Health Surveys (DHS)) covering the entire Cameroon national territory. Results: There were highly significant association between underweight and wasting. For all discriminant statistical methods used, the test error rates (using an independent testing sample) are less than 5%; the Area Under the Curve (AUC) using the Receiver Operating Characteristic (ROC) is 0.86. Conclusions: Weight-for-age can be used for accurately classifying a child whose wasting status is unknown. The result is useful in Cameroon as too often the height measurements may not be feasible, thus the need for estimating wasted children.Anthropometric measures, nutritional status, discriminant analysis, underweight, wasting

Ectopia cordis : a report of two cases in Cameroon

This article reports two cases of ectopia cordis in two children aged one day and twenty months respectively. A one day old newborn had complete thoracic ectopia cordis associated with an internal cardiac defect and severe thoracic and abdominal wall malformations. The centre does not have the facilities to manage complex congenital defects and prior to being transferred to a cardiac centre, the neonate died on the second day of admission. A 20-month old baby had partial ectopia of the heart and a defect in the abdominal wall. He had no major congenital cardiac defect and has remained clinically stable with no life threatening symptomspeer-reviewe

A non-simply laced version for cluster structures on 2-Calabi-Yau categories

This paper investigates a non simply-laced version of cluster structures for 2-Calabi-Yau or stably 2-Calabi-Yau categories over arbitrary fields. It results that 2-Calabi-Yau or stably 2-Calabi-Yau categories having a cluster tilting subcategory with neither loops nor 2-cycles do have the generalized version of cluster structure. This is in particular the case of cluster categories over non-algebraically closed fields.Comment: Beamer version. Journal of Pure and Applied Algebra, Available online 1 December 2013 http://dx.doi.org/10.1016/j.jpaa.2013.11.02

Impacts de la gestion du sol sur la biomasse microbienne et le statut organique deu sol de la zone ouest du Cameroun

Le gouvernement du Cameroun compte doubler les productions agricoles et le volume des exportations d'ici 2015. La plupart des transformations d'intérêt agronomique dans le sol sont d'origine biochimique et se déroulent essentiellement en la présence d'êtres vivants et leurs enzymes. L'objectif de cette étude était d'évaluer les effets de l'utilisation du sol (système de culture et pratiques culturales) sur les propriétés physiques, chimiques et biologiques des sols des agriculteurs. La méthodologie a consisté à observer les pratiques paysannes dans trois localités (NJOGNOM, KOUPARA et BAÏGOM) du bassin de FOUMBOT (ouest du Cameroun) où les pratiques culturales ont peu varié depuis trois ans puis d'y prélever des échantillons de sol sur l'horizon 0-20 cm afin d'en évaluer la biomasse microbienne et les activités des enzymes du sol. Comparés au sol de végétation de type savane arbustive du même bassin, sol n'ayant encore subi d'action anthropique, les sols mis en culture ont présenté des valeurs significativement élevées de carbone organique total (Corg), phosphore soluble (P), pH, conductivité électrique (CE) et des activités de la ?-glucosidase, des déshydrogénases et de la phosphatase acide. Les sols étudiés ont certes présenté une similitude texturale (classe texturale de type argileux) mais ils étaient différents les uns des autres par leurs propriétés microbiologiques, conséquentes de leur mode de gestion. Ainsi, les valeurs de la biomasse microbienne C (Cmic : 312,0 - 544,5 mg Kg-1sol sec), la biomasse microbienne N (Nmic : 5,4 - 25,31 mg Kg-1sol sec) ont diversement varié selon le système de culture. Le rendement microbien (Cmic/Corg) du sol portant la Morelle africaine (Solanum africanum) et la tomate (Lycopersicon esculentum) (site de KOUPARA) et celui portant le haricot vert (Phaseolus vulgaris), la Morelle africaine et la tomate (site de BAÏGOM), respectivement de 0,80 % et 1,35 % étaient faibles comparés à celui du sol témoin (1,60 %) et à celui du sol portant le basilic (Ocimum basilicum L.) et Morelle africaine (1,65 %) (site de NJOGNOM) indiquant une diminution de la concentration des microorganismes du sol selon la pratique culturale. L'analyse des relations entre les différents paramètres a révélé d'une part, une corrélation significative et négative (r = - 0,806 ; P = 0,01) entre le pourcentage d'argile et la biomasse microbienne C, d'autre part un rapport Corg/N très élevé dans le site de KOUPARA (Corg/N : 206 > 100 - 500), suggérant respectivement une répartition hétérogène de la population des microorganismes et une immobilisation du P. La détermination des paramètres physiques, chimiques, enzymatiques et microbiologiques sélectionnés a donc permis donc d'approcher la dynamique d'évolution des sols étudiés.. ( Résumé d'auteur)

Impacts of land use on soil microbial biomass and soil organic status in Western Cameroon

Living organisms and their enzymes are responsible of most of the soil biotic interactions in agro-ecosystems. This study was to evaluate the impact of land use (cropping systems and farming practices) on the physical, chemical and biological properties of soil in Western Cameroon. We sampled soil (0-20 cm) in four different land use systems (extensive, intermediate, intensive and an undisturbed natural habitat). We also measured soil health indicators in each land use type. We observed that the total organic carbon (Corg) and P availability was significantly higher in agro-ecosystems as compared to undisturbed natural habitat. Similar trends were recorded for pH, electrical conductivity, and ?-glucosidase, dehydrogenases and acid phosphatase activities. We also recorded a significant variation in microbial biomass C (Cmic: 312.0 to 544.5 mg kg-1 dry soil) and microbial biomass N (Nmic: 5.40 to 25.31 mg kg-1 dry soil) with land use intensity. The Cmic/Corg ratios of soils were two-fold lower in agricultural lands with vegetables than the undisturbed control plot indicating a decrease in soil microorganisms. Regression analysis revealed a negative correlation (r = - 0.806, P < 0.01) between soil clay content and microbial biomass C, and a high Corg/N ratio suggesting a heterogeneous distribution of the population of microorganisms between soils and the immobilization of P, respectively. Our results suggest that the selected soil health indicators were sensitive to farming practices and cropping systems. (Résumé d'auteur

Association between WHO cut-offs for childhood overweight and obesity and cardiometabolic risk

Abstract Objective To examine the association between cardiovascular risk and childhood overweight and obesity using the BMI cut-offs recommended by the WHO. Design Children were classified as normal weight, overweight and obese according to the WHO BMI-for-age reference. Blood pressure, lipids, glucose, insulin, homeostasis model assessment-insulin resistance (HOMA-IR) and uric acid levels were compared across BMI groups. ANOVA and tests of linearity were used to assess overall mean differences across groups. Crude and adjusted odds ratios were calculated for adverse plasma levels of biochemical variables. Setting Paediatric care centres. Subjects Children (n 149) aged 8-18 years. Results About 37 %, 22 % and 41 % of children were classified respectively as normal weight, overweight and obese. There were significant linear mean differences between BMI groups in systolic blood pressure, HDL-cholesterol, TAG, insulin, HOMA-IR and uric acid. Obese children were 10·6 times more likely than normal-weight children to have hypertension; OR for other associations were 60·2 (high insulin), 39·5 (HOMA-IR), 27·9 (TAG), 16·0 (HDL-cholesterol), 4·3 (LDL-cholesterol) and 3·6 (uric acid). Overweight children were more likely than normal-weight children to have hypertension (OR = 3·5), high insulin (OR = 28·2), high HOMA-IR (OR = 23·3) and high TAG (OR = 16·1). Nearly 92 % and 57 % of the obese and overweight children, respectively, had one or more risk factor. Conclusions Obesity and overweight defined using the WHO BMI-for-age cut-offs identified children with higher metabolic and vascular risk. These results emphasize the importance of prevention of overweight and obesity in childhood to reduce cardiovascular ris

Estimation of a multivariate mean under model selection uncertainty

<p>Model selection uncertainty would occur if we selected a model based on one data set and subsequently applied it for statistical inferences, because the "correct" model would not be selected with certainty. &nbsp;<br />When the selection and inference are based on the same dataset, some additional problems arise due to the correlation of the two stages (selection and inference). In this paper model selection uncertainty is considered and model averaging is proposed. The proposal is related to the theory of James and Stein of estimating more than three parameters from independent normal observations. We suggest that a model averaging scheme taking into account the selection procedure could be more appropriate than model selection alone. Some properties of this model averaging estimator are investigated; in particular we show using Stein's results that it is a minimax estimator and can outperform Stein-type estimators.</p