The Evolution of the Demand for Temporary Help Supply Employment in the United States

The Bureau of Labor Statistics has reported an extraordinary increase in temporary help supply (THS) employment during the late 1980s and the 1990s. However, little is known about the venues where these THS employees actually work. Our estimates indicate that the proportion of THS employees in each major American industry, except the public sector, increased during 1977-97. By 1997, close to 4 percent of the employees in manufacturing and services were THS workers. In the service sector, the increase was accompanied by a large increase in direct hires. In manufacturing, however, it was accompanied by a decline in direct hiring from its peak in 1989 even though output increased substantially in the 1990s. Practically, all of the growth in THS employment is attributed to a change in the hiring behavior of firms, rather than to a disproportional increase in the size of more THS-intensive industries.

Mean Field Theory for Sigmoid Belief Networks

We develop a mean field theory for sigmoid belief networks based on ideas from statistical mechanics. Our mean field theory provides a tractable approximation to the true probability distribution in these networks; it also yields a lower bound on the likelihood of evidence. We demonstrate the utility of this framework on a benchmark problem in statistical pattern recognition---the classification of handwritten digits.Comment: See http://www.jair.org/ for any accompanying file

The role of eco-evolutionary experience in invasion success

Invasion ecology has made considerable progress in identifying specific mechanisms that potentially determine success and failure of biological invasions. Increasingly, efforts are being made to interrelate or even synthesize the growing number of hypotheses in order to gain a more comprehensive and integrative understanding of invasions. We argue that adopting an eco-evolutionary perspective on invasions is a promising approach to achieve such integration. It emphasizes the evolutionary antecedents of invasions, i.e. the species’ evolutionary legacy and its role in shaping novel biotic interactions that arise due to invasions. We present a conceptual framework consisting of five hypothetical scenarios about the influence of so-called ‘eco-evolutionary experience’ in resident native and invading non-native species on invasion success, depending on the type of ecological interaction (predation, competition, mutualism, and commensalism). We show that several major ecological invasion hypotheses, including ‘enemy release’, ‘EICA’, ‘novel weapons’, ‘naive prey’, ‘new associations’, ‘missed mutualisms’ and ‘Darwin’s naturalization hypothesis’ can be integrated into this framework by uncovering their shared implicit reference to the concept of eco-evolutionary experience. We draft a routine for the assessment of eco-evolutionary experience in native and non-native species using a food web-based example and propose two indices (xpFocal index and xpResidents index) for the actual quantification of eco-evolutionary experience. Our study emphasizes the explanatory potential of an eco-evolutionary perspective on biological invasions

Leading indicators of currency crises

The authors examine the empirical evidence on currency crises and propose a specific early-warning system. This system involves monitoring the evolution of several indicators that tend to exhibit unusual behavior in the periods preceding a crisis. An indicator exceeding a certain threshold value should be interpreted as a warning"signal"that a currency crisis may take place within the following 24 months. The threshold values are calculated to strike a balance between the risk of having many false signals and the risk of missing many crises. Within this approach, the variables with the best track record include exports, deviations of the real exchange rate from trend, the ratio of broad money to gross international reserves, output, and equity prices. The evidence does not support some of the other indicators that were considered, including imports, bank deposits,the difference between foreign and domestic real deposit interest rates, and the ratio of lending to deposit interest rates.Payment Systems&Infrastructure,Economic Theory&Research,Fiscal&Monetary Policy,Environmental Economics&Policies,Labor Policies,Economic Theory&Research,Macroeconomic Management,Economic Stabilization,Environmental Economics&Policies,Financial Economics

Calculation of gravitational wave forms from black hole collisions and disk collapse: Applying perturbation theory to numerical spacetimes

Many simulations of gravitational collapse to black holes become inaccurate before the total emitted gravitational radiation can be determined. The main difficulty is that a significant component of the radiation is still in the near-zone, strong field region at the time the simulation breaks down. We show how to calculate the emitted waveform by matching the numerical simulation to a perturbation solution when the final state of the system approaches a Schwarzschild black hole. We apply the technique to two scenarios: the head-on collision of two black holes, and the collapse of a disk to a black hole. This is the first reasonably accurate calculation of the radiation generated from colliding black holes that form from matter collapse.Comment: 8 pages (RevTex 3.0 with 7 uuencoded figures

Species from different taxonomic groups show similar invasion traits

Invasion ecology tends to treat taxonomic groups separately. However, given that all invasive species go through the same stages of the invasion process (transport, escape, establishment, spread), it is likely that – across taxa – comparable traits help to successfully complete this process ("invasion traits"). Perhaps not all invasive species have the same invasion traits, but different combinations of invasion traits can be found among invaders, corresponding to different possibilities to become a successful invader. These combinations of invasion traits might be linked to taxonomic affiliation, but this is not necessarily the case. We created a global dataset with 201 invasive species from seven major taxonomic groups (animals, green plants, fungi, heterokonts, bacteria, red algae, alveolates) and 13 invasion traits that are applicable across all taxa. The dataset was analysed with cluster analysis to search for similarities in combinations of invasion traits. Three of the five clusters, comprising 60% of all species, contain several major taxonomic groups. While some invasion trait frequencies were significantly related to taxonomic affiliation, the results show that invasive species from different taxonomic groups often share similar combinations of invasion traits. A post-hoc analysis suggests that combinations of traits characterizing successful invaders can be associated with invasion stages across taxa. Our findings suggest that there are no universal invasion traits which could explain the invasion success of all invaders, but that invaders are successful for different reasons which are represented by different combinations of invasion traits across taxonomic groups