Borrower Self-Selection, Underwriting Costs, and Subprime Mortgage Credit Supply

In the U.S., households participate in two very different types of credit markets. Personal lending is characterized by continuous risk-based pricing in which lenders offer households a continuous distribution of borrowing possibilities based on estimates of their creditworthiness. This contrasts sharply with mortgage markets where lenders specialize in specific risk categories of borrowers and mortgage supply is stepwise linear. The contrast between continuous lending for personal loans and discrete lending by specialized lenders for mortgage credit has led to concerns regarding the efficiency and equity of mortgage lending. This paper sheds both theoretical and empirical light on the differences in the two credit markets. The theory section demonstrates why, in a perfectly competitive credit market where all lenders have the same underwriting technology, mortgage credit supply curves are stepwise linear and lenders specialize in prime or subprime lending. The empirical section then provides evidence that borrowers are being effectively sorted based on risk characteristics by the market

The effect of a lifted flame on the stability of round fuel jets

International audienceThe stability and dynamics of an axisymmetric lifted flame are studied by means of direct numerical simulation (DNS) and linear stability analysis of the reacting low-Mach-number equations. For light fuels (such as non-premixed methane/air flames), the non-reacting premixing zone upstream of the lifted flame base contains a pocket of absolute instability supporting self-sustaining oscillations, causing flame flicker even in the absence of gravity. The liftoff heights of the unsteady flames are lower than their steady counterparts (obtained by the method of selective frequency damping (SFD)), owing to premixed flame propagation during a portion of each cycle. From local stability analysis, the lifted flame is found to have a significant stabilizing influence at and just upstream of the flame base, which can truncate the pocket of absolute instability. For sufficiently low liftoff heights, the truncated pocket of absolute instability can no longer support self-sustaining oscillations, and the flow is rendered globally stable. © 2008 Cambridge University Press

Highly prevalent but not always persistent: undergraduate and graduate student's misconceptions about psychology.

Although past research has documented the prevalence of misconceptions in introductory psychology classes, few studies have assessed how readily upper-level undergraduate and graduate students endorse erroneous beliefs about the discipline. In Study 1, we administered a 30-item misconception test to an international sample of 670 undergraduate, Master’s and doctoral students. Analyses indicated that participants identified and rejected the majority of misconceptions, with doctoral students performing better than their Master’s or undergraduate peers. In Study 2, we administered a revised version of our questionnaire to a novel sample of 557 students while controlling for number of years spent at university, psychology courses completed and need for cognition. Once again, we found that graduate students rejected more, affirmed less and reported lower levels of uncertainty than their undergraduate counterparts. Educational implications and future research directions are discussed

Twisted absolute instability in lifted flames

International audienceThe theory of resonant modes is extended to finite length systems containing pinch points of complex axial wavenumber k0 and frequency ω0 with arbitrary ω0kk=∂2ω/∂k2. The quantity ω0kk is shown to be an important indicator of how streamwise boundary conditions modify the local absolute mode at (k0,ω0). In particular, when Im(ω0kk)>0, the pinch point is twisted, and resonant modes owing to streamwise boundary conditions may then have growth rates greater than that of the unbounded absolute mode. In this case, global instability may occur while the flow is only convectively unstable. The premixing zone between the nozzle and a lifted flame on a variable-density jet is an example of a streamwise-confined system containing a twisted pinch point. For this system, linear stability analysis is employed to locate resonant modes along a solution curve in the complex k and ω planes. The orientation of the solution curve predicts destabilization owing to streamwise confinement as well as increasing global frequency with decreasing lift-off height as observed in previous direct numerical simulations. The theory also suggests that low-frequency fluctuations observed in the simulations may be explained by beating between two resonant modes of slightly differing frequencies. © 2009 American Institute of Physics

Health Status, Insurance, and Expenditures in the Transition from Work to Retirement

This paper analyzes the dynamics of health insurance coverage, health expenditures, and health status in the decade expanding from 1992 to 2002, for a cohort of older Americans. We follow 13,594 individuals interviewed in Waves 1 to 6 of the Health and Retirement Study, most of whom were born between 1930 and 1940, as they transition from work into retirement. Although this “depression cohort” is by and large fairly well prepared for retirement in terms of pension coverage and savings, we identify significant gaps in their health insurance coverage, especially among the most disadvantaged members of this cohort. We find that government health insurance programs—particularly Medicare and Medicaid—significantly reduce the number of individuals who are uninsured and the risks of large out of pocket health care costs. However, prior to retirement large numbers of these respondents were uninsured, nearly 18% at the first survey in 1992. Moreover, a much larger share, about 55% of this cohort, are transitorily uninsured, that is, they experience one or more spells, lasting from several months to several years, without health insurance coverage. We also identify a much smaller group of persistently uninsured individuals, and show that this group has significantly less wealth, and higher rates of poverty, unemployment, and health problems, disability, and higher mortality rates than the rest of the members of the cohort under study. We provide evidence that lack of health insurance coverage is correlated with reduced utilization of health care services; for example, respondents with no health insurance visit the doctor one fourth as often as those with private insurance and are also more likely to report declines in health status. We also analyze the components of out of pocket health care costs, and show that prescription drug costs constituted a rapidly rising share of the overall cost of health care during the period of analysis.

The Neutral Hydrogen Bridge between M31 and M33

The Green Bank Telescope has been used to search for 21cm HI emission over a large area between the galaxies M31 and M33 in an attempt to confirm at 9.1 arcmin angular resolution the detection by Braun and Thilker (2004) of a very extensive neutral gas "bridge" between the two systems at the level NHI approximately 10^{17} cm^{-2}. We detect HI emission at several locations up to 120 kpc in projected distance from M31, at least half the distance to M33, with velocities similar to that of the galaxies, confirming the essence of the Braun and Thilker discovery. The HI does not appear to be associated with the extraplanar high-velocity clouds of either galaxy. In two places we measure NHI > 3 x 10^{18} cm^{-2}, indicative of concentrations of HI with ~10^5 solar masses on scales <2 kpc, but over most of the field we have only 5sigma upper limits of NHI <= 1.4 x 10^{18} cm^{-2}. In very deep measurements in two directions HI lines were detected at a few 10^{17} cm^{-2}. The absence of emission at another location to a 5sigma limit NHI <= 1.5 x 10^{17} cm^{-2} suggests that the HI bridge is either patchy or confined to within ~125 kpc of M31. The measurements also cover two of M31's dwarf galaxies, And II and And XV, but in neither case is there evidence for associated HI at the 5sigma level of 1.4 x 10^4 solar masses of HI for And II, and 9.3 x 10^3 solar masses for And XV.Comment: Submitted to the Astronomical Journa

The need for handheld computers in the classroom

The handheld computer has become a viable tool for education. It costs less, weighs less, and takes less space in the classroom than a desktop computer. This review of literature shows the merits of using handheld computers in the classroom. In order to appreciate the handheld computers, one needs to research the history of computers and how the handheld computer is being used in the corporate world, as well as by educators. By researching handheld computers past use and current trends, it should be possible to evaluate their impact in the classroom

Reduced-order representation of near-wall structures in the late transitional boundary layer

International audienceDirect numerical simulations (DNS) of controlled H- and K-type transitions to turbulence in an M=0.2 (where M is the Mach number) nominally zero-pressure-gradient and spatially developing flat-plate boundary layer are considered. Sayadi, Hamman & Moin (J. Fluid Mech., vol. 724, 2013, pp. 480-509) showed that with the start of the transition process, the skin-friction profiles of these controlled transitions diverge abruptly from the laminar value and overshoot the turbulent estimation. The objective of this work is to identify the structures of dynamical importance throughout the transitional region. Dynamic mode decomposition (DMD) (Schmid, J. Fluid Mech., vol. 656, 2010, pp. 5-28) as an optimal phase-averaging process, together with triple decomposition (Reynolds & Hussain, J. Fluid Mech., vol. 54 (02), 1972, pp. 263-288), is employed to assess the contribution of each coherent structure to the total Reynolds shear stress. This analysis shows that low-frequency modes, corresponding to the legs of hairpin vortices, contribute most to the total Reynolds shear stress. The use of composite DMD of the vortical structures together with the skin-friction coefficient allows the assessment of the coupling between near-wall structures captured by the low-frequency modes and their contribution to the total skin-friction coefficient. We are able to show that the low-frequency modes provide an accurate estimate of the skin-friction coefficient through the transition process. This is of interest since large-eddy simulation (LES) of the same configuration fails to provide a good prediction of the rise to this overshoot. The reduced-order representation of the flow is used to compare the LES and the DNS results within this region. Application of this methodology to the LES of the H-type transition illustrates the effect of the grid resolution and the subgrid-scale model on the estimated shear stress of these low-frequency modes. The analysis shows that although the shapes and frequencies of the low-frequency modes are independent of the resolution, the amplitudes are underpredicted in the LES, resulting in underprediction of the Reynolds shear stress

Multitrophic diversity effects of network degradation

Predicting the functional consequences of biodiversity loss in realistic, multitrophic communities remains a challenge. No existing biodiversity-ecosystem function study to date has simultaneously incorporated information on species traits, network topology, and extinction across multiple trophic levels, while all three factors are independently understood as critical drivers of post-extinction network structure and function. We fill this gap by comparing the functional consequences of simulated species loss both within (monotrophic) and across (bitrophic) trophic levels, in an ecological interaction network estimated from spatially explicit field data on tropical fecal detritus producer and consumers (mammals and dung beetles). We simulated trait-ordered beetle and mammal extinction separately (monotrophic extinction) and the coextinction of beetles following mammal loss (bitrophic extinction), according to network structure. We also compared the diversity effects of bitrophic extinction models using a standard monotrophic function (the daily production or consumption of fecal detritus) and a unique bitrophic functional metric (the proportion of daily detritus production that is consumed). We found similar mono- and bitrophic diversity effects, regardless of which species traits were used to drive extinctions, yet divergent predictions when different measures of function were used. The inclusion of information on network structure had little apparent effect on the qualitative relationship between diversity and function. These results contribute to our growing understanding of the functional consequences of biodiversity from real systems and underscore the importance of species traits and realistic functional metrics to assessments of the ecosystem impacts of network degradation through species loss