918 resultados para confounding variable
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We report the observation of the insulator-to-metal transition in crystalline silicon samples supersaturated with vanadium. Ion implantation followed by pulsed laser melting and rapid resolidification produce high quality single-crystalline silicon samples with vanadium concentrations that exceed equilibrium values in more than 5 orders of magnitude. Temperature-dependent analysis of the conductivity and Hall mobility values for temperatures from 10K to 300K indicate that a transition from an insulating to a metallic phase is obtained at a vanadium concentration between 1.1 × 10^(20) and 1.3 × 10^(21) cm^(−3) . Samples in the insulating phase present a variable-range hopping transport mechanism with a Coulomb gap at the Fermi energy level. Electron wave function localization length increases from 61 to 82 nm as the vanadium concentration increases in the films, supporting the theory of impurity band merging from delocalization of levels states. On the metallic phase, electronic transport present a dispersion mechanism related with the Kondo effect, suggesting the presence of local magnetic moments in the vanadium supersaturated silicon material.
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The invention relates to a variable-spectrum solar simulator for characterising photovoltaic systems. The simulator can be used to obtain a spectrum adjusted to the solar spectrum, both for a standard spectrum or a real spectrum adjusted to local irradiation conditions. The simulator also allows the spatial-angular characteristics of the sun to be reproduced. The invention comprises: a broad-spectrum light source, the flux from which is emitted through an aperture; an optical system which collimates the primary source; a system which disperses the beam chromatically; an optical system which forms an image of the dispersed primary source at a given position, at which a spatial mask is placed in order to filter the received irradiance spectrally; an optical system which captures the filtered spectrum and returns, mixes and concentrates same in a secondary source with the desired spectral, angular, and spatial characteristics; an optical system which collimates the secondary source such that it reproduces the angular characteristics of the sun; and a control system.
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En línea con la creciente importancia de la cobertura de riesgos, la eficacia de las diferentes estrategias y las posibilidades de mejorar a través de diferentes modelos estocásticos han sido analizados en un gran número de estudios académicos en diferentes campos, activos financieros, energía, petróleo, etc. La controversia en la literatura sobre estrategias de cobertura se concentra en torno a tres aspectos principals. En primer lugar, una cuestión clave es la elección de la medida de eficacia que va a determinar la función de optimizar. Entre las diferentes medidas propuestas, la reducción de la varianza es el enfoque más simple y común. Muchos autores consideran que otros criterios, como diferentes especificaciones de funciones de utilidad, el riesgo de pérdidas o la variación de la rentabilidad ,se deben tomar en cuenta con el fin de medir la efectividad de cobertura. Estos criterios derivan en diferentes medidas de efectividad de cobertura como el equivalente cierto (CE), Valor en Riesgo (VaR), Valor en Riesgo Condicional (CVaR) o Pérdida Esperada (ES), Momentos parciales inferiores (LPM), semi-varianza, etc. . Un examen de las principales aportaciones de la literatura muestra la falta de uniformidad o resultados concluyentes en favor de una u otra estrategia. En segundo lugar, otro tema importante es la evolución temporal de la relación de cobertura. Hay una controversia en la literatura en cuanto a si la cobertura dinámica, utilizando relaciones que incorporan la nueva información que llega al mercado, es superior a relaciones estáticas, como la cobertura unitaria o la de mínimos cuadrados. En tercer lugar, cuando se trata de estimar los ratios de cobertura, se emplean muchas técnicas diferentes, que van desde los modelos más simples a enfoques dinámicos muy complejos. Sin embargo, no está claro que estos modelos complejos mejoren la eficacia global y en nuestra opinión, no resuelven favorablemente el compromiso entre complejidad y efectividad...
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To know how marketing variables affect customer value is essential for a company in order to be market and customer oriented, and to improve investment efficiency in both attracting and retaining customers. Thus, the assessment of the influence of marketing variables in customer value is of prime importance. This is recognized in many empirical studies of these variables, which address the impact of a single variable (or sets of a few variables) on customer value. A comprehensive, integrated assessment of all marketing variables and their interdependencies is an arduous and complex task for researchers and marketing managers. This research proposes a theoretical model of customer value that takes into account all significant marketing variables that have been partially addressed in empirical investigations of other researchers. These marketing variables include brand and reputation, point of sale, employees, price, termination fee commitment, discounts, complementarity of products, experiences, emotions, perceived value, quality, satisfaction, switching costs, and loyalty. The model incorporates the relationship between each variable with retention and with customer value as well as the relationships between them. A special focus is placed on the empirical analysis of the termination fee commitment and its relationship with customer value. This variable is widely used in the telecommunication’s industry for its influence on customer retention from the moment of purchase. However, there is strikingly little research in this topic. A large customer database of a telecommunications company containing five years information about 63.165 customers is used for this purpose. Multivariate linear regression and ANOVA method are applied...
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Las investigaciones relativas al ratio óptimo de cobertura con futuros de una cartera que replica un índice de renta variable se han llevado a cabo hasta la fecha mediante técnicas econométricas. Los modelos aplicados han ido perfeccionando sus especificaciones, pero todos ellos parten de unos supuestos que se alejan del mundo real, ya que no consideran el pago discreto de dividendos y se basan en una serie del futuro de próximo vencimiento que presenta saltos, al estar compuesta por mini-series encadenadas de futuros de distinto vencimiento. Como consecuencia de estas limitaciones, aunque la eficacia de la cobertura de los modelos econométricos puede considerarse satisfactoria en términos generales, se producen errores significativos en algunos puntos de la serie. La principal aportación de esta tesis a la investigación académica y a la práctica profesional de la gestión de carteras es la definición de un Modelo Algebraico de Cobertura (MAC) que no está basado en técnicas econométricas, es más sencillo de aplicar y superior en resultados104 a los modelos econométricos más utilizados hasta la fecha (MCO, ECM y GARCH) 105. El modelo MAC parte del supuesto de mercado eficiente y de la consideración de un horizonte temporal diario de la cobertura...
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BACKGROUND: Epidemiological studies show that high circulating cystatin C is associated with risk of cardiovascular disease (CVD), independent of creatinine-based renal function measurements. It is unclear whether this relationship is causal, arises from residual confounding, and/or is a consequence of reverse causation. OBJECTIVES: The aim of this study was to use Mendelian randomization to investigate whether cystatin C is causally related to CVD in the general population. METHODS We incorporated participant data from 16 prospective cohorts (n ¼ 76,481) with 37,126 measures of cystatin C and added genetic data from 43 studies (n ¼ 252,216) with 63,292 CVD events. We used the common variant rs911119 in CST3 as an instrumental variable to investigate the causal role of cystatin C in CVD, including coronary heart disease, ischemic stroke, and heart failure. RESULTS: Cystatin C concentrations were associated with CVD risk after adjusting for age, sex, and traditional risk factors (relative risk: 1.82 per doubling of cystatin C; 95% confidence interval [CI]: 1.56 to 2.13; p ¼ 2.12 1014). The minor allele of rs911119 was associated with decreased serum cystatin C (6.13% per allele; 95% CI: 5.75 to 6.50; p ¼ 5.95 10211), explaining 2.8% of the observed variation in cystatin C. Mendelian randomization analysis did not provide evidence for a causal role of cystatin C, with a causal relative risk for CVD of 1.00 per doubling cystatin C (95% CI: 0.82 to 1.22; p ¼ 0.994), which was statistically different from the observational estimate (p ¼ 1.6 105 ). A causal effect of cystatin C was not detected for any individual component of CVD. CONCLUSIONS: Mendelian randomization analyses did not support a causal role of cystatin C in the etiology of CVD. As such, therapeutics targeted at lowering circulating cystatin C are unlikely to be effective in preventing CVD.
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Actualmente encontramos una fuerte presión en las organizaciones por adaptarse a un mundo competitivo con un descenso en las utilidades y una incertidumbre constante en su flujo de caja. Estas circunstancias obligan a las organizaciones al mejoramiento continuo buscando nuevas formas de gestionar sus procesos y sus recursos. Para las organizaciones de prestación de servicios en el sector de telecomunicaciones una de las ventajas competitivas más importantes de obtener es la productividad debido a que sus ganancias dependen directamente del número de actividades que puedan ejecutar cada empleado. El reto es hacer más con menos y con mejor calidad. Para lograrlo, la necesidad de gestionar efectivamente los recursos humanos aparece, y aquí es donde los sistemas de compensación toman un rol importante. El objetivo en este trabajo es diseñar y aplicar un modelo de remuneración variable para una empresa de prestación de servicios profesionales en el sector de las telecomunicaciones y con esto aportar al estudio de la gestión del desempeño y del talento humano en Colombia. Su realización permitió la documentación del diseño y aplicación del modelo de remuneración variable en un proyecto del sector de telecomunicaciones en Colombia. Su diseño utilizó las tendencias de programas remunerativos y teorías de gestión de desempeño para lograr un modelo integral que permita el crecimiento sostenido en el largo plazo y la motivación al recurso más importante de la organización que es el talento humano. Su aplicación permitió también la documentación de problemas y aciertos en la implementación de estos modelos.
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The main topic of this thesis is confounding in linear regression models. It arises when a relationship between an observed process, the covariate, and an outcome process, the response, is influenced by an unmeasured process, the confounder, associated with both. Consequently, the estimators for the regression coefficients of the measured covariates might be severely biased, less efficient and characterized by misleading interpretations. Confounding is an issue when the primary target of the work is the estimation of the regression parameters. The central point of the dissertation is the evaluation of the sampling properties of parameter estimators. This work aims to extend the spatial confounding framework to general structured settings and to understand the behaviour of confounding as a function of the data generating process structure parameters in several scenarios focusing on the joint covariate-confounder structure. In line with the spatial statistics literature, our purpose is to quantify the sampling properties of the regression coefficient estimators and, in turn, to identify the most prominent quantities depending on the generative mechanism impacting confounding. Once the sampling properties of the estimator conditionally on the covariate process are derived as ratios of dependent quadratic forms in Gaussian random variables, we provide an analytic expression of the marginal sampling properties of the estimator using Carlson’s R function. Additionally, we propose a representative quantity for the magnitude of confounding as a proxy of the bias, its first-order Laplace approximation. To conclude, we work under several frameworks considering spatial and temporal data with specific assumptions regarding the covariance and cross-covariance functions used to generate the processes involved. This study allows us to claim that the variability of the confounder-covariate interaction and of the covariate plays the most relevant role in determining the principal marker of the magnitude of confounding.
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In collaboration with G.D. SpA I attended an internship with the purpose of developing a filter for the position control of industrial machines during testing and maintenance operations. The filter elaborates a signal in position provided by an electonic handwheel, in order to enable the application to be controlled with a signal in velocity with arbitrarily dynamics chosen during the design phase. Limiting the dynamics of the filter provide a more stable and less demanding reference trajectory which reduce the vibrations and tracking errors of the motor controlled by it. It also prevents misusages of the handwheel from the technician which could end up in harmful interferences between the mechanical parts moved by the handwheel.
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A miniaturised gas analyser is described and evaluated based on the use of a substrate-integrated hollow waveguide (iHWG) coupled to a microsized near-infrared spectrophotometer comprising a linear variable filter and an array of InGaAs detectors. This gas sensing system was applied to analyse surrogate samples of natural fuel gas containing methane, ethane, propane and butane, quantified by using multivariate regression models based on partial least square (PLS) algorithms and Savitzky-Golay 1(st) derivative data preprocessing. The external validation of the obtained models reveals root mean square errors of prediction of 0.37, 0.36, 0.67 and 0.37% (v/v), for methane, ethane, propane and butane, respectively. The developed sensing system provides particularly rapid response times upon composition changes of the gaseous sample (approximately 2 s) due the minute volume of the iHWG-based measurement cell. The sensing system developed in this study is fully portable with a hand-held sized analyser footprint, and thus ideally suited for field analysis. Last but not least, the obtained results corroborate the potential of NIR-iHWG analysers for monitoring the quality of natural gas and petrochemical gaseous products.
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Radiotherapy (RT) is a risk factor for accelerated carotid artery atherosclerotic disease in subjects with head and neck cancer. However, the risk factors of RT-induced carotid artery remodeling are not established. This study aimed to investigate the effects of RT on carotid and popliteal arteries in subjects with head and neck cancer and to evaluate the relationship between baseline clinical and laboratory features and the progression of RT-induced atherosclerosis. Eleven men (age = 57.9 ± 6.2years) with head and neck cancer who underwent cervical bilateral irradiation were prospectively examined by clinical and laboratory analysis and by carotid and popliteal ultrasound before and after treatment (mean interval between the end of RT and the post-RT assessment = 181 ± 47 days). No studied subject used hypocholesterolemic medications. Significant increases in carotid intima-media thickness (IMT) (0.95 ± 0.08 vs. 0.87 ± 0.05 mm; p < 0.0001) and carotid IMT/diameter ratio (0.138 ± 0.013 vs. 0.129 ± 0.014; p = 0.001) were observed after RT, while no changes in popliteal structural features were detected. In addition, baseline low-density lipoprotein cholesterol levels showed a direct correlation with RT-induced carotid IMT change (r = 0.66; p = 0.027), while no other studied variable exhibited a significant relationship with carotid IMT change. These results indicate that RT-induced atherosclerosis is limited to the irradiated area and also suggest that it may be predicted by low-density lipoprotein cholesterol levels in subjects with head and neck cancer.
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This article analyzes food insecurity and hunger in Brazilian families with children under five years of age. This was a nationally representative cross-sectional study using data from the National Demographic and Health Survey on Women and Children (PNDS-2006), in which the outcome variable was moderate to severe food insecurity, measured by the Brazilian Food Insecurity Scale (EBIA). Prevalence estimates and prevalence ratios were generated with 95% confidence intervals. The results showed a high prevalence of moderate to severe food insecurity, concentrated in the North and Northeast regions (30.7%), in economic classes D and E (34%), and in beneficiaries of conditional cash transfer programs (36.5%). Multivariate analysis showed that the socioeconomic relative risks (beneficiaries of conditional cash transfers), regional relative risks (North and Northeast regions), and economic relative risks (classes D and E) were 1.8, 2.0 and 2.4, respectively. Aggregation of the three risks showed 48% of families with moderate to severe food insecurity, meaning that adults and children were going hungry during the three months preceding the survey.
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This study aimed to identify novel biomarkers for thyroid carcinoma diagnosis and prognosis. We have constructed a human single-chain variable fragment (scFv) antibody library that was selected against tumour thyroid cells using the BRASIL method (biopanning and rapid analysis of selective interactive ligands) and phage display technology. One highly reactive clone, scFv-C1, with specific binding to papillary thyroid tumour proteins was confirmed by ELISA, which was further tested against a tissue microarray that comprised of 229 thyroid tissues, including: 110 carcinomas (38 papillary thyroid carcinomas (PTCs), 42 follicular carcinomas, 30 follicular variants of PTC), 18 normal thyroid tissues, 49 nodular goitres (NG) and 52 follicular adenomas. The scFv-C1 was able to distinguish carcinomas from benign lesions (P=0.0001) and reacted preferentially against T1 and T2 tumour stages (P=0.0108). We have further identified an OTU domain-containing protein 1, DUBA-7 deubiquitinating enzyme as the scFv-binding antigen using two-dimensional polyacrylamide gel electrophoresis and mass spectrometry. The strategy of screening and identifying a cell-surface-binding antibody against thyroid tissues was highly effective and resulted in a useful biomarker that recognises malignancy among thyroid nodules and may help identify lower-risk cases that can benefit from less-aggressive management.
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The objective of this study was to review the growth curves for Turner syndrome, evaluate the methodological and statistical quality, and suggest potential growth curves for clinical practice guidelines. The search was carried out in the databases Medline and Embase. Of 1006 references identified, 15 were included. Studies constructed curves for weight, height, weight/height, body mass index, head circumference, height velocity, leg length, and sitting height. The sample ranged between 47 and 1,565 (total = 6,273) girls aged 0 to 24 y, born between 1950 and 2006. The number of measures ranged from 580 to 9,011 (total = 28,915). Most studies showed strengths such as sample size, exclusion of the use of growth hormone and androgen, and analysis of confounding variables. However, the growth curves were restricted to height, lack of information about selection bias, limited distributional properties, and smoothing aspects. In conclusion, we observe the need to construct an international growth reference for girls with Turner syndrome, in order to provide support for clinical practice guidelines.
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To identify risk factors associated with post-operative temporomandibular joint dysfunction after craniotomy. The study sample included 24 patients, mean age of 37.3 ± 10 years; eligible for surgery for refractory epilepsy, evaluated according to RDC/TMD before and after surgery. The primary predictor was the time after the surgery. The primary outcome variable was maximal mouth opening. Other outcome variables were: disc displacement, bruxism, TMJ sound, TMJ pain, and pain associated to mandibular movements. Data analyses were performed using bivariate and multiple regression methods. The maximal mouth opening was significantly reduced after surgery in all patients (p = 0.03). In the multiple regression model, time of evaluation and pre-operative bruxism were significantly (p < .05) associated with an increased risk for TMD post-surgery. A significant correlation between surgery follow-up time and maximal opening mouth was found. Pre-operative bruxism was associated with increased risk for temporomandibular joint dysfunction after craniotomy.