Efficient designs for mean estimation in multilevel populations and test norming

A crucial step in the research process is the choice of the design of the study because a poorly designed study can have serious consequences for science (e.g. biased or unreliable results) and society (e.g. a waste of resources or bad decisions in health and education based on invalid research conclusions). This thesis deals with the design of two types of studies: surveys for mean estimation in multilevel populations (e.g. estimation of average alcohol consumption by students grouped in schools), and normative studies for estimating reference values for psychological test scores and questionnaires (e.g. to measure patients’ symptoms). Both types of studies are of practical importance: results from surveys can help policymakers, and reference values are used by clinicians or educators to assess individuals. Thus, averages and reference values must be estimated with the highest possible precision, but without wasting resources (i.e. time and money). Hence, the main objective of this thesis is to provide guidelines for planning both types of studies to achieve precise estimates using minimum resources

Crystal growth from a supersaturated melt: relaxation of the solid-liquid dynamic stiffness

We discuss the growth process of a crystalline phase out of a metastable over-compressed liquid that is brought into contact with a crystalline substrate. The process is modeled by means of molecular dynamics. The particles interact via the Lennard-Jones potential and their motion is locally thermalized by Langevin dynamics. We characterize the relaxation process of the solid-liquid interface, showing that the growth speed is maximal for liquid densities above the solid coexistence density, and that the structural properties of the interface rapidly converge to equilibrium-like properties. In particular, we show that the off-equilibrium dynamic stiffness can be extracted using capillary wave theory arguments, even if the growth front moves fast compared to the typical diffusion time of the compressed liquid, and that the dynamic stiffness converges to the equilibrium stiffness in times much shorter than the diffusion time

Generation of sub-ion scale magnetic holes from electron shear flow instabilities in plasma turbulence

Magnetic holes (MHs) are coherent structures associated with strong magnetic field depressions in magnetized plasmas. They are observed in many astrophysical environments at a wide range of scales but their origin is still under debate. In this work we investigate the formation of sub-ion scale MHs using a fully kinetic 2D simulation of plasma turbulence initialized with parameters typical of the Earth's magnetosheath. Our analysis shows that the turbulence is capable of generating sub-ion scale MHs from large scale fluctuations via the following mechanism: first, the nonlinear large scale dynamics spontaneously leads to the development of thin and elongated electron velocity shears; these structures then become unstable to the electron Kelvin-Helmholtz instability and break up into small scale electron vortices; the electric current carried by these vortices locally reduces the magnetic field, inducing the formation of sub-ion scale MHs. The MHs thus produced exhibit features consistent with satellite observations and with previous numerical studies. We finally discuss the kinetic properties of the observed sub-ion scale MHs, showing that they are characterized by complex non-Maxwellian electron velocity distributions exhibiting anisotropic and agyrotropic features.Comment: Submitted to AP