11,349 research outputs found
Does grade retention affect achievement? Some evidence from PISA
Grade retention practices are at the forefront of the educational debate. In this paper, we use PISA 2009 data for Spain to measure the effect of grade retention on students achievement. One important problem when analyzing this question is that school outcomes and the propensity to repeat a grade are likely to be determined simultaneously. We address this problem by estimating a Switching Regression Model. We Â…find that grade retention has a negative impact on educational outcomes, but we confiÂ…rm the importance of endogenous selection, which makes observed differences between repeaters and non-repeaters appear 14.6% lower than they actually are. The effect on PISA scores of repeating is much smaller (-10% of non-repeaters average) than the counterfactual reduction that non-repeaters would suffer had they been retained as repeaters (-24% of their average). Furthermore, those who repeated a grade during primary education suffered more than those who repeated a grade of secondary school, although the effect of repeating at both times is, as expected, much larger.Grade retention, educational scores, PISA
Mixing across fluid interfaces compressed by convective flow in porous media
We study the mixing in the presence of convective flow in a porous medium.
Convection is characterized by the formation of vortices and stagnation points,
where the fluid interface is stretched and compressed enhancing mixing. We
analyze the behavior of the mixing dynamics in different scenarios using an
interface deformation model. We show that the scalar dissipation rate, which is
related to the dissolution fluxes, is controlled by interfacial processes,
specifically the equilibrium between interface compression and diffusion, which
depends on the flow field configuration. We consider different scenarios of
increasing complexity. First, we analyze a double-gyre synthetic velocity
field. Second, a Rayleigh-B\'enard instability (the Horton-Rogers-Lapwood
problem), in which stagnation points are located at a fixed interface. This
system experiences a transition from a diffusion controlled mixing to a chaotic
convection as the Rayleigh number increases. Finally, a Rayleigh-Taylor
instability with a moving interface, in which mixing undergoes three different
regimes: diffusive, convection dominated, and convection shutdown. The
interface compression model correctly predicts the behavior of the systems. It
shows how the dependency of the compression rate on diffusion explains the
change in the scaling behavior of the scalar dissipation rate. The model
indicates that the interaction between stagnation points and the correlation
structure of the velocity field is also responsible for the transition between
regimes. We also show the difference in behavior between the dissolution fluxes
and the mixing state of the systems. We observe that while the dissolution flux
decreases with the Rayleigh number, the system becomes more homogeneous. That
is, mixing is enhanced by reducing diffusion. This observation is explained by
the effect of the instability patterns
Does grade retention affect achievement? Some evidence from Pisa
Grade retention practices are at the forefront of the educational debate. In this paper, we use PISA 2009 data for Spain to measure the effect of grade retention on students’achievement. One important problem when analyzing this question is that school outcomes and the propensity to repeat a grade are likely to be determined simultaneously. We address this problem by estimating a Switching Regression Model. We find that grade retention has a negative impact on educational outcomes, but we confirm the importance of endogenous selection, which makes observed differences between repeaters and non-repeaters appear 14.6% lower than they actually are. The effect on PISA scores of repeating is much smaller (-10% of non-repeaters’average) than the counterfactual reduction that non-repeaters would suffer had they been retained as repeaters (-24% of their average). Furthermore, those who repeated a grade during primary education suffered more than those who repeated a grade of secondary school, although the effect of repeating at both times is, as expected, much larger.Grade retention, educational scores, PISA
Implications for New Physics from Fine-Tuning Arguments: II. Little Higgs Models
We examine the fine-tuning associated to electroweak breaking in Little Higgs
scenarios and find it to be always substantial and, generically, much higher
than suggested by the rough estimates usually made. This is due to implicit
tunings between parameters that can be overlooked at first glance but show up
in a more systematic analysis. Focusing on four popular and representative
Little Higgs scenarios, we find that the fine-tuning is essentially comparable
to that of the Little Hierarchy problem of the Standard Model (which these
scenarios attempt to solve) and higher than in supersymmetric models. This does
not demonstrate that all Little Higgs models are fine-tuned, but stresses the
need of a careful analysis of this issue in model-building before claiming that
a particular model is not fine-tuned. In this respect we identify the main
sources of potential fine-tuning that should be watched out for, in order to
construct a successful Little Higgs model, which seems to be a non-trivial
goal.Comment: 39 pages, 26 ps figures, JHEP forma
Soft interactions in jet quenching
We study the collisional aspects of jet quenching in a high energy nuclear
collision, especially in the final state pion gas. The jet has a large energy,
and acquires momentum transverse to its axis more effectively by multiple soft
collisions than by few hard scatterings (as known from analogous systems such
as J/\psi production at Hera). Such regime of large E and small momentum
transfer corresponds to Regge kinematics and is characteristically dominated by
the pomeron. From this insight we estimate the jet quenching parameter in the
hadron medium (largely a pion gas) at the end of the collision, which is
naturally small and increases with temperature in line with the gas density.
The physics in the quark-gluon plasma/liquid phase is less obvious, and here we
revisit a couple of simple estimates that suggest indeed that the
pomeron-mediated interactions are very relevant and should be included in
analysis of the jet quenching parameter. Finally, the ocasional hard collisions
produce features characteristic of a L\`evy flight in the q_perp^2 plane
perpendicular to the jet axis. We suggest one- and two-particle q_perp
correlations as interesting experimental probes.Comment: 14 pages, 16 figure
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