7,790 research outputs found
Multilevel modelling of refusal and noncontact nonresponse in household surveys: evidence from six UK government surveys
This paper analyses household unit nonresponse and interviewer effects in six major UK government surveys using a multilevel multinomial modelling approach. The models are guided by current conceptual frameworks and theories of survey participation. One key feature of the analysis is the investigation of survey dependent and independent effects of household and interviewer characteristics, providing an empirical exploration of the leverage-salience theory. The analysis is based on the 2001 UK Census Link Study, a unique data source containing an unusually rich set of auxiliary variables, linking the response outcome of six surveys to census data, interviewer observation data and interviewer information, available for respondents and nonrespondents
Alternative approaches to multilevel modelling of survey noncontact and refusal
We review three alternative approaches to modelling survey noncontact and refusal: multinomial, sequential and sample selection (bivariate probit) models. We then propose a multilevel extension of the sample selection model to allow for both interviewer effects and dependency between noncontact and refusal rates at the household and interviewer level. All methods are applied and compared in an analysis of household nonresponse in the UK, using a dataset with unusually rich information on both respondents and nonrespondents from six major surveys. After controlling for household characteristics, there is little evidence of residual correlation between the unobserved characteristics affecting noncontact and refusal propensities at either the household or the interviewer level. We also find that the estimated coefficients of the multinomial and sequential models are surprisingly similar, which further investigation via a simulation study suggests is due to there being little overlap between the predictors of noncontact and refusal
Optimal Dynamic Procurement Policies for a Storable Commodity with L\'evy Prices and Convex Holding Costs
In this paper we study a continuous time stochastic inventory model for a
commodity traded in the spot market and whose supply purchase is affected by
price and demand uncertainty. A firm aims at meeting a random demand of the
commodity at a random time by maximizing total expected profits. We model the
firm's optimal procurement problem as a singular stochastic control problem in
which controls are nondecreasing processes and represent the cumulative
investment made by the firm in the spot market (a so-called stochastic
"monotone follower problem"). We assume a general exponential L\'evy process
for the commodity's spot price, rather than the commonly used geometric
Brownian motion, and general convex holding costs.
We obtain necessary and sufficient first order conditions for optimality and
we provide the optimal procurement policy in terms of a "base inventory"
process; that is, a minimal time-dependent desirable inventory level that the
firm's manager must reach at any time. In particular, in the case of linear
holding costs and exponentially distributed demand, we are also able to obtain
the explicit analytic form of the optimal policy and a probabilistic
representation of the optimal revenue. The paper is completed by some computer
drawings of the optimal inventory when spot prices are given by a geometric
Brownian motion and by an exponential jump-diffusion process. In the first case
we also make a numerical comparison between the value function and the revenue
associated to the classical static "newsvendor" strategy.Comment: 28 pages, 3 figures; improved presentation, added new results and
section
Contraceptive confidence and timing of first birth in Moldova: an event history analysis of retrospective data
Objectives: To test the contraceptive confidence hypothesis in a modern context. The hypothesis is that women using effective or modern contraceptive methods have increased contraceptive confidence and hence a shorter interval between marriage and first birth than users of ineffective or traditional methods. We extend the hypothesis to incorporate the role of abortion, arguing that it acts as a substitute for contraception in the study context.Setting: Moldova, a country in South-East Europe. Moldova exhibits high use of traditional contraceptive methods and abortion compared with other European countries.Participants: Data are from a secondary analysis of the 2005 Moldovan Demographic and Health Survey, a nationally representative sample survey. 5377 unmarried women were selected.Primary and secondary outcome measures: The outcome measure was the interval between marriage and first birth. This was modelled using a piecewise-constant hazard regression, with abortion and contraceptive method types as primary variables along with relevant sociodemographic controls.Results: Women with high contraceptive confidence (modern method users) have a higher cumulative hazard of first birth 36?months following marriage (0.88 (0.87 to 0.89)) compared with women with low contraceptive confidence (traditional method users, cumulative hazard: 0.85 (0.84 to 0.85)). This is consistent with the contraceptive confidence hypothesis. There is a higher cumulative hazard of first birth among women with low (0.80 (0.79 to 0.80)) and moderate abortion propensities (0.76 (0.75 to 0.77)) than women with no abortion propensity (0.73 (0.72 to 0.74)) 24?months after marriage.Conclusions: Effective contraceptive use tends to increase contraceptive confidence and is associated with a shorter interval between marriage and first birth. Increased use of abortion also tends to increase contraceptive confidence and shorten birth duration, although this effect is non-linear—women with a very high use of abortion tend to have lengthy intervals between marriage and first birth
The spectrum of the Broad Line Region and the high-energy emission of powerful blazars
High-energy emission (from the X-ray through the gamma-ray band) of Flat
Spectrum Radio Quasars is widely associated with the inverse Compton (IC)
scattering of ambient photons, produced either by the accretion disk or by the
Broad Line Region, by high-energy electrons in a relativistic jet. In the
modelling of the IC spectrum one usually adopts a simple black-body
approximation for the external radiation field, though the real shape is
probably more complex. The knowledge of the detailed spectrum of the external
radiation field would allow to better characterize the soft-medium X-ray IC
spectrum, which is crucial to address several issues related to the study of
these sources. Here we present a first step in this direction, calculating the
IC spectra expected by considering a realistic spectrum for the external
radiation energy density produced by the BLR, as calculated with the
photoionization code CLOUDY. We find that, under a wide range of the physical
parameters characterizing the BLR clouds, the IC spectrum calculated with the
black-body approximation reproduces quite well the exact spectrum for energies
above few keV. In the soft energy band, instead, the IC emission calculated
using the BLR emission shows a complex shape, with a moderate excess with
respect to the approximate spectrum, which becomes more important for
decreasing values of the peak frequency of the photoionizing continuum. We also
show that the high-energy spectrum shows a marked steepening, due to the energy
dependence of the scattering cross section, above a characteristic energy of
10-20 GeV, quasi independent on the Lorentz factor of the jet.Comment: 10 pages, 9 figures, accepted for publication in MNRA
The correlates of natural method use in Moldova: is natural method use associated with poverty and isolation?
Natural method use is often associated with high levels of unwanted births and induced abortions. This study investigates the correlates of natural method use in Moldova, a country with one of the highest proportions of natural contraceptive users in Europe. We hypothesize that economic and spatial disadvantage increase the reliance on natural methods whereas exposure to FP programs decreases the probability of natural method use. The analysis considers a sub-sample of 5860 sexually-active women from the 2005 Demographic and Health Survey. Results from multilevel multinomial models, controlling for relevant characteristics and data structure, show that economic disadvantage increases the probability of natural method use; but the overall effect is small. Higher FP media exposure reduces natural method use; however this effect attenuates with age. We conclude that FP efforts directed towards the poorest may have limited impact, but interventions targeted at older women could reduce the burden of unwanted pregnancies
Spontaneous nucleation of structural defects in inhomogeneous ion chains
Structural defects in ion crystals can be formed during a linear quench of
the transverse trapping frequency across the mechanical instability from a
linear chain to the zigzag structure. The density of defects after the sweep
can be conveniently described by the Kibble-Zurek mechanism. In particular, the
number of kinks in the zigzag ordering can be derived from a time-dependent
Ginzburg-Landau equation for the order parameter, here the zigzag transverse
size, under the assumption that the ions are continuously laser cooled. In a
linear Paul trap the transition becomes inhomogeneous, being the charge density
larger in the center and more rarefied at the edges. During the linear quench
the mechanical instability is first crossed in the center of the chain, and a
front, at which the mechanical instability is crossed during the quench, is
identified which propagates along the chain from the center to the edges. If
the velocity of this front is smaller than the sound velocity, the dynamics
becomes adiabatic even in the thermodynamic limit and no defect is produced.
Otherwise, the nucleation of kinks is reduced with respect to the case in which
the charges are homogeneously distributed, leading to a new scaling of the
density of kinks with the quenching rate. The analytical predictions are
verified numerically by integrating the Langevin equations of motion of the
ions, in presence of a time-dependent transverse confinement. We argue that the
non-equilibrium dynamics of an ion chain in a Paul trap constitutes an ideal
scenario to test the inhomogeneous extension of the Kibble-Zurek mechanism,
which lacks experimental evidence to date.Comment: 19 pages, 5 figure
- …