Two-Method Planned Missing Designs for Longitudinal Research

We examine longitudinal extensions of the two-method measurement design, which uses planned missingness to optimize cost-efficiency and validity of hard-to-measure constructs. These designs use a combination of two measures: a “gold standard” that is highly valid but expensive to administer, and an inexpensive (e.g., survey-based) measure that contains systematic measurement bias (e.g., response bias). Using simulated data on four measurement occasions, we compared the cost-efficiency and validity of longitudinal designs where the gold standard is measured at one or more measurement occasions. We manipulated the nature of the response bias over time (constant, increasing, fluctuating), the factorial structure of the response bias over time, and the constraints placed on the latent variable model. Our results showed that parameter bias is lowest when the gold standard is measured on at least two occasions. When a multifactorial structure was used to model response bias over time, it is necessary to have the “gold standard” measures included at every time point, in which case most of the parameters showed low bias. Almost all parameters in all conditions displayed high relative efficiency, suggesting that the 2-method design is an effective way to reduce costs and improve both power and accuracy in longitudinal research

Exploring Student, Family, and School Predictors of Self-Determination Using NLTS2 Data

This study conducted secondary analysis of data from the National Longitudinal Transition Study-2 (NLTS2) to examine the degree to which student, family, and school constructs predicted self-determination outcomes. Multi-group structural equation modeling was used to examine predictive relationships between 5 student, 4 family, and 7 school constructs developed from NLTS2 data and self-determination outcomes (autonomy, psychological empowerment, and self-realization) across disability groups. The pattern of predictive relationship between the constructs and self-determination outcomes across disability groups was complex. Only one construct—self-concept—showed a positive predictive relationship with all three self-determination constructs across most disability groups. Implications of the complex pattern of findings for research and practice are discussed

Method and apparatus for receiving and tracking phase modulated signals

An apparatus and technique are described for receiving and tracking analog or digital phase modulated signals from 0 deg to 360 deg phase shift. In order to track a signal with many phases, a detector discerns the phase modulation of the incoming signal and a phase shifter generates a negative phase shift opposite in angle to the detected phase angle. This produces a converted series sideband component barrier signal. The residual carrier signal and the converted series sideband component carrier are added together to produce a tracking carrier signal. The tracking carrier signal is multiplied with the output from a voltage controlled oscillator in the tracking loop to obtain an error signal which drives the voltage controlled oscillator and tracks the incoming signal frequency. The technique is less susceptible to carrier interference which may degrade tracking and tracking may be performed at lower signal to noise ratios and for lower input signal power levels

Applications of Geographic Information Systems as an analytical and visualization tool for mass real estate valuation : a case study of Fontibón district, Bogotá, Colombia

Los avalúos masivos de propiedades están basados en su mayoría en modelos estadísticos globales que tienen en cuenta las características de las propiedades; sin embargo, los modelos utilizados en Bogotá, no tienen en cuenta variables de localización explícitamente. En este estudio se utilizaron Sistemas de Información Geográfica (SIG) como herramienta analítica para desarrollar una serie de variables de localización y analizar su impacto en el valor de las propiedades; se crearon variables como la proximidad a un aeropuerto, proximidad a centros comerciales y el acceso a vías principales, se generaron mapas de superficie con los valores de las propiedades y se analizó el efecto de las diferentes variables de localización en el precio de las propiedades. Cuatro diferentes enfoques se utilizaron con el fin de analizar el efecto de las variables de localización: coeficiente de Pearson, regresión lineal múltiple, step-wise y se utilizaron modelos locales de estadística (Regresión Geográfica Ponderada o Geographically Weighted Regression GWR) para analizar los comportamientos no estacionarios de las variables. El SIG como herramienta de visualización demostró ser útil en la construcción de mapas de valor, siendo además fundamental para mostrar e interpretar los resultados de los modelos estadísticos locales. Se encontró que en el área de estudio existe una considerable variación de los parámetros, identificando grupos de tendencias, esto hace que el uso de modelos locales de estadística sea ideal. Los modelos locales mostraron que los valores estadísticos en los grupos de predios más costosos parecen estar bien explicadas por las variables de localización, pero en contraste, para los grupos de predios menos costosos, las variables de localización tienen menos capacidad para explicar la varianza en los valores de los predios.The majority of approaches for mass appraisal are based on global statistical models that take into account property characteristics, and, in developing countries, location variables are not often explicitly considered. The present study uses Geographic Information Systems (GIS) as an analytical tool to develop a number of location variables and to analyze their impact in the value of properties. Variables like proximity to an airport, proximity to leisure facilities, proximity to shopping centres, and access to main roads, among others, are created, and property values estimated. Surface value maps are created and the effect of the different location variables in the price is hypothesized. Four different statistical approaches are used in order to analyze the effect of proximity attributes: bivariate Pearson coefficient, multiple linear regression, step-wise procedure, and local spatial statistics (Geographically Weighted Regression-GWR) are used to analyze spatially non-stationary behaviours. The visualization capabilities of GIS prove helpful in constructing value maps, and GIS also proves to be fundamental to report and interpret the results of local spatial statistical models. The results reveal that the spatial variation of some of the parameters is significant and that there are clusters of tendencies. Given this strong clustered distribution, the use of local spatial statistics, to analyze variations of the different statistical parameters across the study area, is ideal. The local statistical results show that prices in expensive clusters seem to be well explained by location factors, unlike prices in less expensive clusters that are poorly explained. The main conclusion from this thesis is that, for the study area, low-priced properties are more difficult to replicate with proximity attributes than high-priced properties

Exploring Essential Characteristics of Self-Determination for Diverse Students Using Data From NLTS2

This study explored the impact of race/ethnicity on three of the four essential characteristics of self-determination—autonomy, self-realization, and psychological empowerment—directly assessed in the National Longitudinal Transition Study-2. Specifically, the impact of race/ethnicity was examined with six disability groups established in previous research: high incidence disabilities (learning disabilities, emotional disturbances, speech language impairments, and other health impairments), sensory disabilities (visual and hearing impairments), cognitive disabilities (autism, multiple disabilities, and deaf-blindness); intellectual disability, traumatic brain injury, and orthopedic impairments. Measurement equivalence was established across groups, but significant differences in the latent means, variances, and covariances were found suggesting a complex pattern of differences based on race/ethnicity within disability groups. Implications for future research and practice are discussed

Structure of self-assembled Mn atom chains on Si(001)

Mn has been found to self-assemble into atomic chains running perpendicular to the surface dimer reconstruction on Si(001). They differ from other atomic chains by a striking asymmetric appearance in filled state scanning tunneling microscopy (STM) images. This has prompted complicated structural models involving up to three Mn atoms per chain unit. Combining STM, atomic force microscopy and density functional theory we find that a simple necklace-like chain of single Mn atoms reproduces all their prominent features, including their asymmetry not captured by current models. The upshot is a remarkably simpler structure for modelling the electronic and magnetic properties of Mn atom chains on Si(001).Comment: 5 pages, 4 figure

Electronic coupling between Bi nanolines and the Si(001) substrate: An experimental and theoretical study

Atomic nanolines are one dimensional systems realized by assembling many atoms on a substrate into long arrays. The electronic properties of the nanolines depend on those of the substrate. Here, we demonstrate that to fully understand the electronic properties of Bi nanolines on clean Si(001) several different contributions must be accounted for. Scanning tunneling microscopy reveals a variety of different patterns along the nanolines as the imaging bias is varied. We observe an electronic phase shift of the Bi dimers, associated with imaging atomic p-orbitals, and an electronic coupling between the Bi nanoline and neighbouring Si dimers, which influences the appearance of both. Understanding the interplay between the Bi nanolines and Si substrate could open a novel route to modifying the electronic properties of the nanolines.Comment: 6 pages (main), 2 pages (SI), accepted by Phys. Rev.

Brownian motion meets Riemann curvature

The general covariance of the diffusion equation is exploited in order to explore the curvature effects appearing on brownian motion over a d-dimensional curved manifold. We use the local frame defined by the so called Riemann normal coordinates to derive a general formula for the mean-square geodesic distance (MSD) at the short-time regime. This formula is written in terms of $O(d)$ invariants that depend on the Riemann curvature tensor. We study the n-dimensional sphere case to validate these results. We also show that the diffusion for positive constant curvature is slower than the diffusion in a plane space, while the diffusion for negative constant curvature turns out to be faster. Finally the two-dimensional case is emphasized, as it is relevant for the single particle diffusion on biomembranes.Comment: 16 pages and 3 figure