Unobserved Heterogeneity in the Binary Logit Model with Cross-Sectional Data and Short Panels: A Finite Mixture Approach

This paper proposes a new approach to dealing with unobserved heterogeneity in applied research using the binary logit model with cross-sectional data and short panels. Unobserved heterogeneity is particularly important in non-linear regression models such as the binary logit model because, unlike in linear regression models, estimates of the effects of observed independent variables are biased even when omitted independent variables are uncorrelated with the observed independent variables. We propose an extension of the binary logit model based on a finite mixture approach in which we conceptualize the unobserved heterogeneity via latent classes. Simulation results show that our approach leads to considerably less bias in the estimated effects of the independent variables than the standard logit model. Furthermore, because identification of the unobserved heterogeneity is weak when the researcher has cross-sectional rather than panel data, we propose a simple approach that fixes latent class weights and improves identification and estimation. Finally, we illustrate the applicability of our new approach using Canadian survey data on public support for redistribution.binary logit model; unobserved heterogeneity; latent classes; simulation

Simulation af Vejtrafik

Hvad gør man, når de eksisterende trafikmodeller af ligevægtstypen ikke længere har deres gyldighed f.eks. pga. trængselsproblemer? Hvad når man ønsker evaluere et informatikprojekt som rampedosering eller hvilken indflydelse nye færdselsregler kan have på kørselsmønstret? Hvad gør man, når man vil informere trafikanter gennem en korttidsprognose om, hvornår kødannelser er afviklet?En måde at besvare disse spørgsmål på er at opbygge og implementere en mikroskopisk simulationsmodel dvs. en model, der simulerer hver eneste bil i vejnettet.TRANSFORSK-projektet baserer sig på en trafiksimuleringsmodel kaldet cellular automata (CA), der også er kendt som Nagel-Schreckenberg modellen efter de to pionerer indenfor feltet. Der er tale om en relativt simpel simulationsmodel, hvor man diskretiserer alle veje sådan, at et vejstykke består af et antal baner, der er opdelt i celler af en hvis størrelse. Disse vejceller kan enten være tomme eller de kan indeholde en bil. Tiden er også diskretiseret i skridt af en vis varighed. I hvert af disse tidsskridt skal alle biler vælge en hastighed de vil køre med (givet ved et antal celler pr. tidskridt) betinget af deres aktuelle hastighed og bilerne umiddelbart omkring dem. Ligeledes skal bilerne tage stilling til om de vil skifte vejbane. Alle skift mellem cellerne sker da ved hop.Den simple model muliggør, at simulationen kan køre hurtigere end realtid. Dette er meget vigtigt, hvis man f.eks. vil lave prognoser for, hvor hurtigt en kø vil være afviklet.Projektet implementerer en CA-trafiksimulator, der er fleksibel i sin variation af diskretiseringsgraden. Det muliggør en undersøgelse af tradeoff mellem hastighed og præcision ved forskellige detaljeringsgrader i den underliggende model

Simulation af Vejtrafik

Hvad gør man, når de eksisterende trafikmodeller af ligevægtstypen ikke længere har deres gyldighed f.eks. pga. trængselsproblemer? Hvad når man ønsker evaluere et informatikprojekt som rampedosering eller hvilken indflydelse nye færdselsregler kan have på kørselsmønstret? Hvad gør man, når man vil informere trafikanter gennem en korttidsprognose om, hvornår kødannelser er afviklet?En måde at besvare disse spørgsmål på er at opbygge og implementere en mikroskopisk simulationsmodel dvs. en model, der simulerer hver eneste bil i vejnettet.TRANSFORSK-projektet baserer sig på en trafiksimuleringsmodel kaldet cellular automata (CA), der også er kendt som Nagel-Schreckenberg modellen efter de to pionerer indenfor feltet. Der er tale om en relativt simpel simulationsmodel, hvor man diskretiserer alle veje sådan, at et vejstykke består af et antal baner, der er opdelt i celler af en hvis størrelse. Disse vejceller kan enten være tomme eller de kan indeholde en bil. Tiden er også diskretiseret i skridt af en vis varighed. I hvert af disse tidsskridt skal alle biler vælge en hastighed de vil køre med (givet ved et antal celler pr. tidskridt) betinget af deres aktuelle hastighed og bilerne umiddelbart omkring dem. Ligeledes skal bilerne tage stilling til om de vil skifte vejbane. Alle skift mellem cellerne sker da ved hop.Den simple model muliggør, at simulationen kan køre hurtigere end realtid. Dette er meget vigtigt, hvis man f.eks. vil lave prognoser for, hvor hurtigt en kø vil være afviklet.Projektet implementerer en CA-trafiksimulator, der er fleksibel i sin variation af diskretiseringsgraden. Det muliggør en undersøgelse af tradeoff mellem hastighed og præcision ved forskellige detaljeringsgrader i den underliggende model

Field clearance of an intertidal bivalve bed: relative significance of the co-occurring blue mussel Mytilus edulis and Pacific oyster Crassostrea gigas

At an approximately 12000 m2 sheltered intertidal bivalve bed in the western part of the Limfjord, Denmark, the Pacific oyster Crassostrea gigas co-occurs with the blue mussel Mytilus edulis. The relative significance of the impact of the 2 species on phytoplankton density during a tidal cycle was estimated by combining field measurements of clearance rates and modelling of the bivalve bed (topography, biomass distribution, temporal and spatial water coverage and depth). The average density of C. gigas and M. edulis was 35 ± 36 and 1001 ± 685 ind. m-2, respectively. The water volume cleared during a tidal cycle was estimated at 45838 m3, of which C. gigas and M. edulis contributed 9169 and 36669 m3, respectively. Therefore, M. edulis contributed 4 times as much as C. gigas to the bivalve bed’s clearance, and the 2 bivalves were estimated to clear the water volume 1.9 times during each tidal cycle. However, the estimated water column cleared during low tide is overestimated due to phytoplankton depletion. Hence, it is concluded that the bivalve bed clears the water close to 1 time each tidal cycle. This, together with a low dry weight of soft parts, indicates that the bivalve bed, in general, is food-limited

Influenza A virus H10N7 detected in dead harbor seals (Phoca vitulina) at several locations in Denmark 2014.

Influenza A virus (IAV) affects a wide range of species, though waterfowl is regarded the natural host for most IAV subtypes. Avian influenza (AI) viruses replicate in the intestinal tract of birds and are mainly transmitted by the fecal-oral route. Pinnipeds share the same shoreline habitats as many waterfowl species and are therefore potentially exposed to AIV. Outbreaks of AI in seals have been described in North America and Asia but prior to 2014 never in Europe. In 2014 massive deaths of harbor seals (Phoca vitulina) were reported in Northern Europe. In Denmark, harbor seals were initially found dead on the Danish island Anholt in Kattegat, which is the sea surrounded by Denmark, Norway and Sweden. Between June and August, 152 harbor seals were found dead. Four seals were submitted to the National Veterinary Institute in Dennmark and diagnosed with severe pneumonia. Influenza A virus of the subtype H10N7 was detected in two out of four seals. Subsequently IAV was detected in dead harbor seals at several locations in Denmark. The IAV outbreak appeared to move with time to the west through the Limfjord to the North Sea and further down south along the west coast of Jutland to the Wadden Sea. Outbreaks were subsequently reported from Germany and The Netherlands. The aim of this study was to characterize the viruses detected at the several locations by molecular and phylogenetic analysis. All viruses were subtyped as H10N7 with genes of avian origin. The HA and NA genes of the viruses were highly similar to H10N7 IAV detected in harbor seals in Sweden in the spring of 2014 and in Germany in the autumn of 2014, suggesting that the same strain of virus had spread from Sweden to Denmark and further on to Germany

Holistic monitoring of freshwater and terrestrial vertebrates by camera trapping and environmental DNA

The anthropogenic impact on the world's ecosystems is severe and the need for non-invasive, cost-effective tools for monitoring and understanding those impacts are therefore urgent. Here, we combine two such methods in a comprehensive multi-year study; camera trapping (CT) and analysis of environmental DNA (eDNA), in river marginal zones of a temperate, wetland Nature Park in Denmark. CT was performed from 2015 to 2019 for a total of 8778 camera trap days and yielded 24,376 animal observations. The CT observations covered 87 taxa, of which 78 were identified to species level, and 73 were wild native species. For eDNA metabarcoding, a total of 114 freshwater samples were collected from eight sites in all four seasons from 2017 to 2018. The eDNA results yielded a total detection of 80 taxa, of which 74 were identified to species level, and 65 were wild native species. While the number of taxa detected with the two methods were comparable, the species overlap was only 20%. In combination, CT and eDNA monitoring thus yielded a total of 115 wild species (20 fishes, 4 amphibians, one snake, 23 mammals, and 67 birds), representing half of the species found via conventional surveys over the last ca. 20 years (83% of fishes, 68% of mammals, 67% of amphibians, 41% of birds, and 20% of reptiles). Our study demonstrates that a holistic approach combining two non-invasive methods, CT, and eDNA metabarcoding, has great potential as a cost-effective biomonitoring tool for vertebrates

SHARQnet – Sophisticated harmonic artifact reduction in quantitative susceptibility mapping using a deep convolutional neural network

Quantitative susceptibility mapping (QSM) reveals pathological changes in widespread diseases such as Parkinson's disease, Multiple Sclerosis, or hepatic iron overload. QSM requires multiple processing steps after the acquisition of magnetic resonance imaging (MRI) phase measurements such as unwrapping, background field removal and the solution of an ill-posed field-to-source-inversion. Current techniques utilize iterative optimization procedures to solve the inversion and background field correction, which are computationally expensive and lead to suboptimal or over-regularized solutions requiring a careful choice of parameters that make a clinical application of QSM challenging. We have previously demonstrated that a deep convolutional neural network can invert the magnetic dipole kernel with a very efficient feed forward multiplication not requiring iterative optimization or the choice of regularization parameters. In this work, we extended this approach to remove background fields in QSM. The prototype method, called SHARQnet, was trained on simulated background fields and tested on 3T and 7T brain datasets. We show that SHARQnet outperforms current background field removal procedures and generalizes to a wide range of input data without requiring any parameter adjustments. In summary, we demonstrate that the solution of ill-posed problems in QSM can be achieved by learning the underlying physics causing the artifacts and removing them in an efficient and reliable manner and thereby will help to bring QSM towards clinical applications

Scattering Theory of Photon-Assisted Electron Transport

The scattering matrix approach to phase-coherent transport is generalized to nonlinear ac-transport. In photon-assisted electron transport it is often only the dc-component of the current that is of experimental interest. But ac-currents at all frequencies exist independently of whether they are measured or not. We present a theory of photon-assisted electron transport which is charge and current conserving for all Fourier components of the current. We find that the photo-current can be considered as an up- and down-conversion of the harmonic potentials associated with the displacement currents. As an example explicit calculations are presented for a resonant double barrier coupled to two reservoirs and capacitively coupled to a gate. Two experimental situations are considered: in the first case the ac-field is applied via a gate, and in the second case one of the contact potentials is modulated. For the first case we show that the relative weight of the conduction sidebands varies with the screening properties of the system. In contrast to the non-interacting case the relative weights are not determined by Bessel functions. Moreover, interactions can give rise to an asymmetry between absorption and emission peaks. In the contact driven case, the theory predicts a zero-bias current proportional to the asymmetry of the double barrier. This is in contrast to the discussion of Tien and Gordon which, in violation of basic symmetry principles, predicts a zero-bias current also for a symmetric double barrier.Comment: 15 pages, 6 figures, REVTE