1,737 research outputs found
Necessity and Opportunity Entrepreneurs and their Duration in Self-employment: Evidence from German Micro Data
Using data from the German Socio-Economic Panel Study (GSOEP), we analyze whether necessity entrepreneurs differ from opportunity entrepreneurs in terms of self-employment duration. Using univariate statistics, we find that opportunity entrepreneurs remain in self-employment longer than necessity entrepreneurs. However, after controlling for the entrepreneursâ education in the professional area where they start their venture, this effect is no longer significant. We therefore conclude that the difference observed is not an original effect but rather is due to selection. We then go on to discuss the implications of our findings for entrepreneurship-policy making, and give suggestions to improve governmental start-up programs
Are Education and Entrepreneurial Income Endogenous and Do Family Background Variables Make Sense as Instruments?: A Bayesian Analysis
Education is a well-known driver of (entrepreneurial) income. The measurement of its influence, however, suffers from endogeneity suspicion. For instance, ability and occupational choice are mentioned as driving both the level of (entrepreneurial) income and of education. Using instrumental variables can provide a way out. However, three questions remain: whether endogeneity is really present, whether it matters and whether the selected instruments make sense. Using Bayesian methods, we find that the relationship between education and entrepreneurial income is indeed endogenous and that the impact of endogeneity on the estimated relationship between education and income is sizeable. We do so using family background variables and show that relaxing the strict validity assumption of these instruments does not lead to strongly different results. This is an important finding because family background variables are generally strongly correlated with education and are available in most datasets. Our approach is applicable beyond the field of returns to education for income. It applies wherever endogeneity suspicion arises and the three questions become relevant.Education, income, entrepreneurship, self-employment, endogeneity, instrumental variables, Bayesian analysis, family background variables
The macroeconomic effects of exogenous fiscal policy shocks in Germany: a disaggregated SVAR analysis
We investigate the short-term effects of fiscal policy shocks on the German economy following the SVAR approach by Blanchard and Perotti (2002). We find that direct government expenditure shocks increase output and private consumption on impact with low statistical significance, while they decrease private investment, though insignificantly. For the sub-category government investment â in contrast to government consumption â a positive output effect is found, which is statistically significant until 12 quarters ahead. Allowing for anticipation effects of fiscal policy does not change the sign of the positive consumption response. Anticipated expenditure shocks have significant effects on output when the shock is realized, but not in the period of anticipation. In sum, effects of expenditure shocks are only short-lived. Government net revenue shocks do not affect output with statistical significance. However, when splitting up this aggregate, direct taxes lower output significantly, while small indirect tax revenue shocks have little effects. Compensation of public employees is equally not effective in stimulating the economy. --Fiscal policy,government spending,net revenue,policy anticipation,structural vector autoregression
Identifying Heavy-Flavor Jets Using Vectors of Locally Aggregated Descriptors
Jets of collimated particles serve a multitude of purposes in high energy
collisions. Recently, studies of jet interaction with the quark-gluon plasma
(QGP) created in high energy heavy ion collisions are of growing interest,
particularly towards understanding partonic energy loss in the QGP medium and
its related modifications of the jet shower and fragmentation. Since the QGP is
a colored medium, the extent of jet quenching and consequently, the transport
properties of the medium are expected to be sensitive to fundamental properties
of the jets such as the flavor of the parton that initiates the jet.
Identifying the jet flavor enables an extraction of the mass dependence in
jet-QGP interactions. We present a novel approach to tagging heavy-flavor jets
at collider experiments utilizing the information contained within jet
constituents via the \texttt{JetVLAD} model architecture. We show the
performance of this model in proton-proton collisions at center of mass energy
GeV as characterized by common metrics and showcase its
ability to extract high purity heavy-flavor jet sample at various jet momenta
and realistic production cross-sections including a brief discussion on the
impact of out-of-time pile-up. Such studies open new opportunities for future
high purity heavy-flavor measurements at jet energies accessible at current and
future collider experiments.Comment: 18 pages, 6 figures and 3 tables. Accepted by JINS
Crystal distribution patterns and their anisotropy behaviour in igneous rocks: towards an automated quantification, first results
Since approximately two decades fractal
geometry offers tools for the quantification
of rock fabrics, and new
methods are currently under development
to investigate the inhomogeneity
of crystal distributions, grain- and
phase-boundary patterns as well as
their anisotropy behaviour (Kruhl et al.
2004). These methods are now adapted
for automated processing and suitable
to quantify the inhomogeneity and
anisotropy of rock fabrics from macro to
microscale. Applications for quantifying
inhomogeneity are mainly based on the
box-counting and map-counting (Peternell
2002) methods, for anisotropy
behaviour mainly based on modified
Cantor-dust methods and provide fractal
dimensions, fractal-dimension isolines
and azimuthal anisotropies of fractal
dimension (AAD, Volland & Kruhl
2004). For instance, the results provide
information about the local variations of
fabric patterns and their prefer orientation
behaviour at macro and microscale.conferenc
Fluid dynamics of bacterial turbulence
Self-sustained turbulent structures have been observed in a wide range of
living fluids, yet no quantitative theory exists to explain their properties.
We report experiments on active turbulence in highly concentrated 3D
suspensions of Bacillus subtilis and compare them with a minimal fourth-order
vector-field theory for incompressible bacterial dynamics. Velocimetry of
bacteria and surrounding fluid, determined by imaging cells and tracking
colloidal tracers, yields consistent results for velocity statistics and
correlations over two orders of magnitude in kinetic energy, revealing a
decrease of fluid memory with increasing swimming activity and linear scaling
between energy and enstrophy. The best-fit model parameters allow for
quantitative agreement with experimental data.Comment: 5 pages, 4 figure
Mapping functional traits: comparing abundance and presence-absence estimates at large spatial scales
Efforts to quantify the composition of biological communities increasingly focus on functional traits. The composition of communities in terms of traits can be summarized in several ways. Ecologists are beginning to map the geographic distribution of trait-based metrics from various sources of data, but the maps have not been tested against independent data. Using data for birds of the Western Hemisphere, we test for the first time the most commonly used method for mapping community trait composition â overlaying range maps, which assumes that the local abundance of a given species is unrelated to the traits in question â and three new methods that as well as the range maps include varying degrees of information about interspecific and geographic variation in abundance. For each method, and for four traits (body mass, generation length, migratory behaviour, diet) we calculated community-weighted mean of trait values, functional richness and functional divergence. The maps based on species ranges and limited abundance data were compared with independent data on community species composition from the American Christmas Bird Count (CBC) scheme coupled with data on traits. The correspondence with observed community composition at the CBC sites was mostly positive (62/73 correlations) but varied widely depending on the metric of community composition and method used (R2: 5.6Ă10â7 to 0.82, with a median of 0.12). Importantly, the commonly-used range-overlap method resulted in the best fit (21/22 correlations positive; R2: 0.004 to 0.8, with a median of 0.33). Given the paucity of data on the local abundance of species, overlaying range maps appears to be the best available method for estimating patterns of community composition, but the poor fit for some metrics suggests that local abundance data are urgently needed to allow more accurate estimates of the composition of communities
Network of recurrent events for the Olami-Feder-Christensen model
We numerically study the dynamics of a discrete spring-block model introduced
by Olami, Feder and Christensen (OFC) to mimic earthquakes and investigate to
which extent this simple model is able to reproduce the observed spatiotemporal
clustering of seismicty. Following a recently proposed method to characterize
such clustering by networks of recurrent events [Geophys. Res. Lett. {\bf 33},
L1304, 2006], we find that for synthetic catalogs generated by the OFC model
these networks have many non-trivial statistical properties. This includes
characteristic degree distributions -- very similar to what has been observed
for real seismicity. There are, however, also significant differences between
the OFC model and earthquake catalogs indicating that this simple model is
insufficient to account for certain aspects of the spatiotemporal clustering of
seismicity.Comment: 11 pages, 16 figure
Meso-scale turbulence in living fluids
Turbulence is ubiquitous, from oceanic currents to small-scale biological and
quantum systems. Self-sustained turbulent motion in microbial suspensions
presents an intriguing example of collective dynamical behavior amongst the
simplest forms of life, and is important for fluid mixing and molecular
transport on the microscale. The mathematical characterization of turbulence
phenomena in active non-equilibrium fluids proves even more difficult than for
conventional liquids or gases. It is not known which features of turbulent
phases in living matter are universal or system-specific, or which
generalizations of the Navier-Stokes equations are able to describe them
adequately. Here, we combine experiments, particle simulations, and continuum
theory to identify the statistical properties of self-sustained meso-scale
turbulence in active systems. To study how dimensionality and boundary
conditions affect collective bacterial dynamics, we measured energy spectra and
structure functions in dense Bacillus subtilis suspensions in quasi-2D and 3D
geometries. Our experimental results for the bacterial flow statistics agree
well with predictions from a minimal model for self-propelled rods, suggesting
that at high concentrations the collective motion of the bacteria is dominated
by short-range interactions. To provide a basis for future theoretical studies,
we propose a minimal continuum model for incompressible bacterial flow. A
detailed numerical analysis of the 2D case shows that this theory can reproduce
many of the experimentally observed features of self-sustained active
turbulence.Comment: accepted PNAS version, 6 pages, click doi for Supplementary
Informatio
- âŠ