Modeling Choice Among Assortments

In this paper we propose a model for describing consumer decision making among assortments or menus of options from which a single option will be chosen at a later time. Central to the derivation of the model is an assumption that consumers are uncertain about their future preferences. The model captures both the utility of the items within the assortments as well as the flexibility the items offer as a group. We support our model empirically with two laboratory experiments. In the first experiment we test the underlying assumptions. In the second, we compare the predictive validity of our model to that provided by other models suggested in the literature

Bacillary dysentery in African children on the Witwatersrand

'It has .... been shown in the United States and elsewhere that when cases of "diarrhoea, enteritis and dysentery" are carefully studied, the majority appear to be bacillary dysentery.'The investigation to be described was carried out to ascertain the importance of bacillary dysentery in the causation of diarrhoeal disorders among African children in the Johannesburg area. It was found that dysentery organisms were present in less than 20% of these patients

Microscopic Theory for the Markovian Decay of Magnetization Fluctuations in Nanomagnets

We present a microscopic theory for the phonon-driven decay of the magnetization fluctuations in a wide class of nanomagnets where the dominant energy is set by isotropic exchange and/or uniaxial anisotropy. Based on the Zwanzig-Mori projection formalism, the theory reveals that the magnetization fluctuations are governed by a single decay rate $\omega_c$, which we further identify with the zero-frequency portion of the associated self-energy. This dynamical decoupling from the remaining slow degrees of freedom is attributed to a conservation law and the discreteness of the energy spectrum, and explains the omnipresent mono-exponential decay of the magnetization over several decades in time, as observed experimentally. A physically transparent analytical expression for $\omega_c$ is derived which highlights the three specific mechanisms of the slowing down effect which are known so far in nanomagnets.Comment: 7 page

Attitudes to organ donation among some urban South African populations remain unchanged: A cross-sectional study (1993 - 2013)

Background. A 1993 paper in the SAMJ suggested that public attitudes to organ donation in South Africa were positive. However, statistics reveal a decline in the annual number of transplants in this country.Objective. To repeat the 1993 survey as far as possible and determine whether public attitudes to organ donation in some South African populations have changed over the past 20 years.Methods. The 1993 study was replicated in 2012 to generate a current data set. This was compared with the raw data from the 1993 study, and an analysis of percentages was used to determine variations.Results. Generally attitudes to organ donation have not changed since 1993, remaining positive among the study population. However, individuals are significantly more hesitant to consider donating the organs of a relative without being aware of that person’s donation preference. Individuals in the black African study population are currently more willing to donate kidneys than in 1993 (66% v. 81%; p<0.0001), but less willing to donate a heart (64% v. 38%; p<0.0001, a liver (40% v. 34%; p<0.036) and corneas (22% v. 15%, p<0.0059).Conclusions. Publicity campaigns aimed at raising awareness of organ donation should emphasise the importance of sharing donation preferences with one’s family in order to mitigate discomfort about making a decision on behalf of another. These campaigns should be culturally and linguistically sensitive. The study should be repeated in all populations over time to continually gauge attitudes

Massive star formation via high accretion rates and early disk-driven outflows

We present an investigation of massive star formation that results from the gravitational collapse of massive, magnetized molecular cloud cores. We investigate this by means of highly resolved, numerical simulations of initial magnetized Bonnor-Ebert-Spheres that undergo collapse and cooling. By comparing three different cases - an isothermal collapse, a collapse with radiative cooling, and a magnetized collapse - we show that massive stars assemble quickly with mass accretion rates exceeding 10^-3 Msol/yr. We confirm that the mass accretion during the collapsing phase is much more efficient than predicted by selfsimilar collapse solutions, i.e. dM/dt ~ c^3/G. We find that during protostellar assembly the mass accretion reaches 20 - 100 c^3/G. Furthermore, we determined the self-consistent structure of bipolar outflows that are produced in our three dimensional magnetized collapse simulations. These outflows produce cavities out of which radiation pressure can be released, thereby reducing the limitations on the final mass of massive stars formed by gravitational collapse. Moreover, we argue that the extraction of angular momentum by disk-threaded magnetic fields and/or by the appearance of bars with spiral arms significantly enhance the mass accretion rate, thereby helping the massive protostar to assemble more quickly.Comment: 22 pages, 12 figures, aastex style, accepted for publication in ApJ, see http://www.ita.uni-heidelberg.de/~banerjee/publications/MassiveStars.pdf for high resolution figure

Semiclassical corrections to the interaction energy of a hard-sphere Boltzmann gas

Quantum effects in statistical mechanics are important when the thermal wavelength is of the order of, or greater than, the mean interatomic spacing. This is examined at depth taking the example of a hard-sphere Boltzmann gas. Using the virial expansion for the equation of state, it is shown that the interaction energy of a classical hard-sphere gas is exactly zero. When the (second) virial coefficient of such a gas is obtained quantum mechanically, however, the quantum contribution to the interaction energy is shown to be substantial. The importance of the semiclassical corrections to the interaction energy shows up dramatically in such a system.Comment: 9 pages, 3 figures, submitted to Eur. J. Phy

Chaotic motion of space charge wavefronts in semiconductors under time-independent voltage bias

A standard drift-diffusion model of space charge wave propagation in semiconductors has been studied numerically and analytically under dc voltage bias. For sufficiently long samples, appropriate contact resistivity and applied voltage - such that the sample is biased in a regime of negative differential resistance - we find chaos in the propagation of nonlinear fronts (charge monopoles of alternating sign) of electric field. The chaos is always low-dimensional, but has a complex spatial structure; this behavior can be interpreted using a finite dimensional asymptotic model in which the front (charge monopole) positions and the electrical current are the only dynamical variables.Comment: 12 pages, 8 figure