Assessing the validity of road safety evaluation studies by analysing causal chains

This paper discusses how the validity of road safety evaluation studies can be assessed by analysing causal chains. A causal chain denotes the path through which a road safety measure influences the number of accidents. Two cases are examined. One involves chemical de-icing of roads (salting). The intended causal chain of this measure is: spread of salt --> removal of snow and ice from the road surface --> improved friction --> shorter stopping distance --> fewer accidents. A Norwegian study that evaluated the effects of salting on accident rate provides information that describes this causal chain. This information indicates that the study overestimated the effect of salting on accident rate, and suggests that this estimate is influenced by confounding variables the study did not control for. The other case involves a traffic club for children. The intended causal chain in this study was: join the club --> improve knowledge --> improve behaviour --> reduce accident rate. In this case, results are rather messy, which suggests that the observed difference in accident rate between members and non-members of the traffic club is not primarily attributable to membership in the club. The two cases show that by analysing causal chains, one may uncover confounding factors that were not adequately controlled in a study. Lack of control for confounding factors remains the most serious threat to the validity of road safety evaluation studies.     

Theoretical analysis of phase-matched second-harmonic generation and optical parametric oscillation in birefringent semiconductor waveguides

We analyze the phase-matching conditions for second-harmonic generation (SHG) and optical parametric oscillation (OPO) in birefringent nonlinear semiconductor waveguides and apply these results to the model system of ZnGeP(2) on a GaP substrate. The analyses and numerical results show that phase matching can be achieved for OPO and SHG for reasonable guide thicknesses throughout much of the infrared, indicating significant potential applications for nonlinear birefringent waveguides. For the fundamental mode of a relatively thick guide the region of phase matching and the phase-matching angles are similar to those in bulk material. However, the waveguide has the added flexibility that phase-matched coupling can occur between the various modes of the guide. For example, the phase-matching region for SHG can be considerably extended by coupling the pump into the guide in the fundamental, m =, mode and phase matching to the m = mode of the second harmonic. Significantly, the results indicate, among other things, that ZnGeP(2) waveguides with harmonic output in the m = mode can be used for efficient SHG from input radiation in the 9.6-10.6-mum region where bulk efficiencies in this wavelength range are too small to be useful.     

Using the Curriculum Vita to Study the Career Paths of Scientists and Engineers: An Exploratory Assessment

In this paper we assess the utility of the curriculum vita (CV) as a data source for examining the career paths of scientists and engineers. CVs were obtained in response to an email message sent to researchers working in the areas of biotechnology and microelectronics. In addition, a number of CVs were obtained "passively" from a search of the Internet. We discuss the methodological issues and problems of this data collection strategy and the results from an exploratory analysis using OLS regression and event history analysis. In sum, despite difficulties with coding and variation in CV formats, this collection strategy seems to us to hold much promise.     

When birds of a feather flock together and when they do not: Status composition, social dominance orientation, and organizational attractiveness.

Although similarity-attraction notions suggest that similarity--for example, in terms of values, personality, and demography--attracts, the authors found that sometimes demographic similarity attracts and sometimes it repels. Consistent with social dominance theory (J. Sidanius & F. Pratto, 1999), they demonstrated in 3 studies that when prospective employees supported group-based social hierarchies (i.e., were high in social dominance orientation), those in high-status groups were attracted to demographic similarity within an organization, whereas those in low-status groups were repelled by it. An important theoretical implication of the findings is that social dominance theory and traditional similarity-attraction notions together help explain a more complex relationship between demographic similarity and attraction than was previously acknowledged in the organizational literature. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Minimizing expected waiting in a medical appointment system

An appointment policy is established for a specific outpatient system, with the goal of reducing a combination of patients' expected waiting times and doctor's expected overtime. Service times are stochastic, and patients are assumed to be punctual if they show for their appointments. Substantial reductions in waiting times can be obtained over traditional scheduling practices, and the procedure is easily implemented by a naive scheduler once the optimal policy is established.     

Damping estimation using free decays and ambient vibration tests

This paper studies the quality of modal damping ratios estimates based on ambient and free vibration tests using both numerical simulations and data collected on large Civil Engineering structures where both tests were performed. The simulated data allowed to study the influence of factors like non-proportional damping or proximity of natural frequencies on the quality of the estimates and also to illustrate the influence of the identification algorithms parameters on the accuracy of the results. The analysis of data collected on a cable-stayed bridge, on the suspended roof of a stadium and on a footbridge permitted to compare the results of both testing approaches in real applications where some factors that cannot be realistically included in the numerical simulations can play an important role. The processing of ambient vibration responses is performed with two output-only identification approaches: frequency domain decomposition and stochastic subspace identification methods. The numerically simulated and the measured free decays were analyzed with a simple method based on filters and fitting of exponential decays and also with the use of subspace models.     

A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts

Catalytic partial oxidation of methane has been reviewed with an emphasis on the reaction mechanisms over transition metal catalysts. The thermodynamics and aspects related to heat and mass transport is also evaluated, and an extensive table on research contributions to methane partial oxidation over transition metal catalysts in the literature is provided.Presented are both theoretical and experimental evidence pointing to inherent differences in the reaction mechanism over transition metals. These differences are related to methane dissociation, binding site preferences, the stability of OH surface species, surface residence times of active species and contributions from lattice oxygen atoms and support species.Methane dissociation requires a reduced metal surface, but at elevated temperatures oxides of active species may be reduced by direct interaction with methane or from the reaction with H, H₂, C or CO.The comparison of elementary reaction steps on Pt and Rh illustrates that a key factor to produce hydrogen as a primary product is a high activation energy barrier to the formation of OH. Another essential property for the formation of H₂ and CO as primary products is a low surface coverage of intermediates, such that the probability of O–H, OH–H and CO–O interactions are reduced.The local concentrations of reactants and products change rapidly through the catalyst bed. This influences the reaction mechanisms, but the product composition is typically close to equilibrated at the bed exit temperature.     

Rational cubic spirals

We consider the problem of finding parametric rational Bézier cubic spirals (planar curves of monotonic curvature) that interpolate end conditions consisting of positions, tangents and curvatures. Rational cubics give more design flexibility than polynomial cubics for creating spirals, making them suitable for many applications. The problem is formulated to enable the numerical robustness and efficiency of the solution-algorithm which is presented and analyzed.