6,044 research outputs found
Modelling count data with overdispersion and spatial effects
In this paper we consider regression models for count data allowing for overdispersion in a Bayesian framework. We account for unobserved heterogeneity in the data in two ways. On the one hand, we consider more flexible models than a common Poisson model allowing for overdispersion in different ways. In particular, the negative binomial and the generalized Poisson distribution are addressed where overdispersion is modelled by an additional model parameter. Further, zero-inflated models in which overdispersion is assumed to be caused by an excessive number of zeros are discussed. On the other hand, extra spatial variability in the data is taken into account by adding spatial random effects to the models. This approach allows for an underlying spatial dependency structure which is modelled using a conditional autoregressive prior based on Pettitt et al. (2002). In an application the presented models are used to analyse the number of invasive meningococcal disease cases in Germany in the year 2004. Models are compared according to the deviance information criterion (DIC) suggested by Spiegelhalter et al. (2002) and using proper scoring rules, see for example Gneiting and Raftery (2004). We observe a rather high degree of overdispersion in the data which is captured best by the GP model when spatial effects are neglected. While the addition of spatial effects to the models allowing for overdispersion gives no or only little improvement, a spatial Poisson model is to be preferred over all other models according to the considered criteria
A default prior for regression coefficients
When the sample size is not too small, M-estimators of regression
coefficients are approximately normal and unbiased. This leads to the familiar
frequentist inference in terms of normality-based confidence intervals and
p-values. From a Bayesian perspective, use of the (improper) uniform prior
yields matching results in the sense that posterior quantiles agree with
one-sided confidence bounds. For this, and various other reasons, the uniform
prior is often considered objective or non-informative. In spite of this, we
argue that the uniform prior is not suitable as a default prior for inference
about a regression coefficient in the context of the bio-medical and social
sciences. We propose that a more suitable default choice is the normal
distribution with mean zero and standard deviation equal to the standard error
of the M-estimator. We base this recommendation on two arguments. First, we
show that this prior is non-informative for inference about the sign of the
regression coefficient. Secondly, we show that this prior agrees well with a
meta-analysis of 50 articles from the MEDLINE database
Modelling workplace contact networks: the effects of organizational structure, architecture, and reporting errors on epidemic predictions
Face-to-face social contacts are potentially important transmission routes
for acute respiratory infections, and understanding the contact network can
improve our ability to predict, contain, and control epidemics. Although
workplaces are important settings for infectious disease transmission, few
studies have collected workplace contact data and estimated workplace contact
networks. We use contact diaries, architectural distance measures, and
institutional structures to estimate social contact networks within a Swiss
research institute. Some contact reports were inconsistent, indicating
reporting errors. We adjust for this with a latent variable model, jointly
estimating the true (unobserved) network of contacts and duration-specific
reporting probabilities. We find that contact probability decreases with
distance, and research group membership, role, and shared projects are strongly
predictive of contact patterns. Estimated reporting probabilities were low only
for 0-5 minute contacts. Adjusting for reporting error changed the estimate of
the duration distribution, but did not change the estimates of covariate
effects and had little effect on epidemic predictions. Our epidemic simulation
study indicates that inclusion of network structure based on architectural and
organizational structure data can improve the accuracy of epidemic forecasting
models.Comment: 36 pages, 4 figure
Likely oscillatory motions of stochastic hyperelastic solids
Stochastic homogeneous hyperelastic solids are characterised by strain-energy
densities where the parameters are random variables defined by probability
density functions. These models allow for the propagation of uncertainties from
input data to output quantities of interest. To investigate the effect of
probabilistic parameters on predicted mechanical responses, we study radial
oscillations of cylindrical and spherical shells of stochastic incompressible
isotropic hyperelastic material, formulated as quasi-equilibrated motions where
the system is in equilibrium at every time instant. Additionally, we study
finite shear oscillations of a cuboid, which are not quasi-equilibrated. We
find that, for hyperelastic bodies of stochastic neo-Hookean or Mooney-Rivlin
material, the amplitude and period of the oscillations follow probability
distributions that can be characterised. Further, for cylindrical tubes and
spherical shells, when an impulse surface traction is applied, there is a
parameter interval where the oscillatory and non-oscillatory motions compete,
in the sense that both have a chance to occur with a given probability. We
refer to the dynamic evolution of these elastic systems, which exhibit inherent
uncertainties due to the material properties, as `likely oscillatory motions'
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The costs of adjusting labor: Evidence from temporally disaggregated data
I estimate the costs for establishments of hires and separations using a dynamic labor demand framework and matched employer-employee data from Germany, which records the exact dates of start and end of an employment spell. I estimate adjustment costs under different assumptions of adjustment frequencies. Under the assumption that establishments revise their labor demand every month, GMM estimates suggest hiring costs per employee of approximately 5,000 Euros, and costs of separations of 1,000 Euros. Hiring costs vary considerably between skilled (8,000 to 28,000 Euros per hire) and unskilled (4,000 to 8,000 Euros) labor. Spatial aggregation (large establishments) is associated with lower cost estimates, and only monthly adjustment frequencies yield estimates consistent with theoretical predictions
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