. We show that an extragalactic jet with a velocity shear gives rise to Fermi like acceleration process for photons scattering withing the shear layers of the jet. Such photons then gain energy to produce a high energy power law. These power law spectra at high energies are frequently observed in several extragalactic objects such as Gamma Ray Bursts (GRBs). We implement the model on GRBs to show that the obtained range of the photon indices are well within their observed values. The analytic results are confirmed with numerical simulations following Monte Carlo approach.

Popper’s ‘Situational Analysis’ (SA) constitutes his methodological proposal for the social sciences. We claim that the two hallmarks of SA are: (i) that scientists assume they possess a ‘wider’ view of the problem-situation than actors do, and (ii) use the model as an ideal ‘benchmark’ scenario to identify the deviation of actors’ actual behaviour from the former. We argue that SA is not a generalization of the neoclassical theory of individual behaviour but captures instead the methodology adopted by modern behavioural economists. Last, we argue that SA highlights a way of acquiring knowledge that has gone unnoticed in the literature.

Rawls ignited a debate in political theory when he introduced a division between the ideal and nonideal parts of a theory of justice. In the ideal part of the theory, one presents a positive conception of justice in a setting that assumes perfect compliance with the rules of justice. In the nonideal part, one addresses the question of what happens under departures from compliance. Critics of Rawls have attacked his focus on ideal theory as a form of utopianism, and have argued that political theory should be focused instead on providing solutions to the manifest injustices of the real world. In this essay, I offer a defense of the ideal/nonideal theory distinction according to which it amounts to nothing more than a division of labor, and explore some scientific analogies. Rawls’s own focus on the ideal part of the theory, I argue, stems from a felt need to clarify the foundations of justice, rather than a utopian neglect of real world problems.

Collaboration networks provide a method for examining the highly heterogeneous structure of collaborative communities. However, we still have limited theoretical understanding of how individual heterogeneity relates to network heterogeneity. The model presented here provides a framework linking an individual's skill set to her position in the collaboration network, and the distribution of skills in the population to the structure of the collaboration network as a whole. This model suggests that there is a non-trivial relationship between skills and network position: individuals with a useful combination of skills will have a disproportionate number of links in the network. Indeed, in some cases, an individual's degree is non-monotonic in the number of skills she has—an individual with very few skills may outperform an individual with many. Special cases of the model suggest that the degree distribution of the network will be skewed, even when the distribution of skills is uniform in the population. The degree distribution becomes more skewed as problems become more difficult, leading to a community dominated by a few high-degree superstars. This has striking implications for labor market outcomes in industries where production is largely the result of collaborative effort.

Background: The public health response to an influenza pandemic or other large-scale health emergency may include mass prophylaxis using multiple points of dispensing (PODs) to deliver countermeasures rapidly to affected populations. Computer models created to date to determine “optimal” staffing levels at PODs typically assume stable patient demand for service. The authors investigated POD function under dynamic and uncertain operational environments.

Methods: The authors constructed a Monte Carlo simulation model of mass prophylaxis (the Dynamic POD Simulator, or D-PODS) to assess the consequences of nonstationary patient arrival patterns on POD function under a variety of POD layouts and staffing plans. Compared are the performance of a standard POD layout under steady-state and variable patient arrival rates that may mimic real-life variation in patient demand.

Results: To achieve similar performance, PODs functioning under nonstationary patient arrival rates require higher staffing levels than would be predicted using the assumption of stationary arrival rates. Furthermore, PODs may develop severe bottlenecks unless staffing levels vary over time to meet changing patient arrival patterns. Efficient POD networks therefore require command and control systems capable of dynamically adjusting intra- and inter-POD staff levels to meet demand. In addition, under real-world operating conditions of heightened uncertainty, fewer large PODs will require a smaller total staff than many small PODs to achieve comparable performance.

Conclusions: Modeling environments that capture the effects of fundamental uncertainties in public health disasters are essential for the realistic evaluation of response mechanisms and policies. D-PODS quantifies POD operational efficiency under more realistic conditions than have been modeled previously. The authors’ experiments demonstrate that effective POD staffing plans must be responsive to variation and uncertainty in POD arrival patterns. These experiments highlight the need for command and control systems to be created to manage emergency response successfully. (Disaster Med Public Health Preparedness. 2009;3(Suppl 2):S121–S131)

Theories of superior collicular and hippocampal function have remarkable similarities. Both structures have been repeatedly implicated in spatial and attentional behaviour and in inhibitory control of locomotion. Moreover, they share certain electrophysiological properties in their single unit responses and in the synchronous appearance and disappearance of slow wave activity. Both are phylogenetically old and the colliculus projects strongly to brainstem nuclei instrumental in the generation of theta rhythm in the hippocampal EEC

On the other hand, close inspection of behavioural and electrophysiological data reveals disparities. In particular, hippocampal processing mainly concerns stimulus ambiguity, contextual significance, and spatial relations or other subtle, higher order characteristics. This requires the use of largely preprocessed sensory information and mediation of poststimulus investigation. Although collicular activity must also be integrated with that of “higher” centres (probably to a varying degree, depending on the nature of stimuli being processed and the task requirements), its primary role in attention is more “peripheral” and specific in controlling orienting/localisation via eye and body movements toward egocentrically labelled spatial positions. In addition, the colliculus may exert a nonspecific influence in alerting higher centres to the imminence of information potentially worthy of focal attention. Nevertheless, it is noteworthy that collicular and hippocampal lesions produce deficits on similar tasks, although the type of deficit is usually different (often opposite) in each case. Functional overlap between hippocampus and colliculus (i.e., strategically synchronised or mutually interdependent activity) is virtually certain vis-à-vis stimulus sampling, for example in the acquisition of information via vibrissal movements and visual scanning. In addition, insofar as stimulus significance is a factor in collicular orienting mechanisms, the hippocampus — cingulate – cortex — colliculus pathway may play a significant role, modulating collicular responsiveness and thus ensuring an attentional strategy appropriate to current requirements (stimulus familiarity, stage of learning). A tentative “reciprocal loop” model is proposed which bridges physiological and behavioural levels of analysis and which would account for the observed degree and nature of functional overlap between the superior colliculus and hippocampus.

In light of growing demands for public accountability, the broadening scope of health technology assessment organizations (HTAOs) activities and their increasing role in decision-making underscore the importance for them to demonstrate their performance. Based on Parson's social action theory, we propose a conceptual model that includes four functions an organization needs to balance to perform well: (i) goal attainment, (ii) production, (iii) adaptation to the environment, and (iv) culture and values maintenance. From a review of the HTA literature, we identify specific dimensions pertaining to the four functions and show how they relate to performance. We compare our model with evaluations reported in the scientific and gray literature to confirm its capacity to accommodate various evaluation designs, contexts of evaluation, and organizational models and perspectives. Our findings reveal the dimensions of performance most often assessed and other important ones that, hitherto, remain unexplored. The model provides a flexible and theoretically grounded tool to assess the performance of HTAOs.

A number of Radiotherapy Departments have extended the working day on linear accelerators, as a method of increasing treatment capacity. However, reports from the Royal College of Radiologists predict a rate of increase in requirements for radiotherapy, significantly in excess of the rate at which radiographers will become available to run the necessary equipment.

Based on the premise that radiographers are the most difficult staff group to recruit, a number of different patterns of radiotherapy linear accelerator use has been investigated, with an aim of seeing which delivers the greatest return on radiographer time. The requirement for radiographers per linac-hour has been examined for a number of lengths of day and utilising various staffing assumptions.

Given current patterns of work, the greatest manpower efficiency in use of trained radiographers is achieved if machines are run for a 7-h-day or for an 8-h-day if part-time radiographers are available. This working pattern relies on all ancillary tasks being removed from the treatment unit, many of which can be performed by other appropriately trained, non-radiographer, staff groups.

With linac-hour demand projected to rise faster than the rate at which new trained radiographers become available, extended working days are not sustainable without significant change to current work patterns.

Vasculogenesis and angiogenesis are two different mechanisms for bloodvessel formation. Angiogenesis occurs when new vessels sprout from pre-existing vasculature in response to external chemical stimuli. Vasculogenesis occurs via the reorganization of randomly distributed cells into a blood vessel network. Experimental modelsof vasculogenesis have suggested that the cells exert traction forcesonto the extracellular matrix and that these forces may playan important role in the network forming process.In order to study the role of the mechanical and chemical forcesin both of these stages of blood vessel formation, we present amathematical model which assumes that (i) cells exert traction forcesonto the extracellular matrix, (ii) the matrix behaves as a linearviscoelastic material, (iii) the cells move along gradients ofexogenously supplied chemical stimuli (chemotaxis) and (iv) these stimuli diffuse or are uptaken by the cells.We study the equations numerically, present an appropriate finite difference scheme and simulate the formation of vascular networks in a plane. Our results compare very well with experimental observations and suggest that spontaneous formation of networks can be explained via a purely mechanical interaction betweencells and the extracellular matrix. We find that chemotaxis alone is not a sufficient force to stimulate formation of pattern. Moreover, duringvessel sprouting, we find that mechanical forces can help in the formationof well defined vascular structures.