551 research outputs found
The Role of Specific Subjects in Education Production Functions: Evidence from Morning Classes in Chicago Public High Schools
Absences in Chicago Public High Schools are 3-7 days per year higher in first period than at other times of the day. This study exploits this empirical regularity and the essentially random variation between students in the ordering of classes over the day to measure how the returns to classroom learning vary by course subject, and how much attendance in one class spills over into learning in other subjects. We find that having a class in first period reduces grades in that course and has little effect on long-term grades or grades in related subjects. We also find moderately-sized negative effects of having a class in first period on test scores in that subject and in related subjects, particularly for math classes.education production, subject-specific, math, English, morning classes, first period, course schedule, quasi-experimental, attendance, absenteeism, Chicago, high school
Customers’ Perceptions of Best Available Hotel Rates
A study of 153 travelers in three airports (Dallas, Pittsburgh, and Ithaca) found that respondents held a generally negative view of hotel companies\u27 pricing policies. The respondents were asked specifically about their reaction to best available rate pricing, as compared to the common practice of quoting a single rate for a multiple-night stay. Compared to the single-rate policy, respondents judged that being charged the lowest possible rate for each night is more fair, more acceptable, more reasonable, and more honest, even if the room rate changes from night to night. In particular, infrequent travelers (those staying in hotels twenty nights per year or less) judged best rate pricing most favorably. In contrast, frequent travelers were essentially indifferent to the two pricing approaches
Best-available-rate Pricing at Hotels: A Study of Customer Perceptions and Reactions
Variable pricing, or demand-based pricing, is a popular revenue management technique by which hotel managers set different nightly rates for the same room based on expected room demand. Operational policies and procedures associated with variable pricing may be confusing to customers, especially if they are not familiar with the practice. Best-available-rate (BAR) pricing is an attempt to reduce that confusion and to guarantee that the guest is quoted the lowest available rate for each night of a multiple-night stay. As a result, instead of paying the same price for each room-night, the guest would pay different prices each night. Understanding customers\u27 perceptions of a BAR policy can help hotel managers better apply revenue management tools that maximize revenue without compromising guest satisfaction. This study surveyed 153 travelers to measure their reactions to BAR pricing and their perception of its fairness, acceptability, reasonableness, and honesty. We found that for a multiple-night stay, customers prefer to be quoted individual rates for each night (non-blended rates) rather than the average price per night over the stay (blended rates). Overall, customers found individual rates to be significantly more fair, acceptable, reasonable, and honest than blended rates. However, customer reactions to non-blended and blended rates differed between frequent and infrequent travelers. Frequent travelers found no difference in fairness between blended and non-blended rates while infrequent travelers perceived non-blended rates to be more favorable. The findings of this study can help managers more precisely tailor the way that they give rates and information to customers during the reservation process. Respondents preferred to be quoted individual rates, so that they know they are paying the lowest available nightly rates, rather than blended rates, which conceal the actual nightly rates. To ensure that customers have positive perceptions of price fairness and honesty, managers should quote non-blended rates, such as those that accompany BAR guarantees. Managers should also pay close attention to the implementation of a BAR guarantee policy, as the poor execution of a complex variable-pricing policy could compromise its acceptance
The velocity dispersion and mass profile of the Milky Way
We re-analyse the velocity-dispersion profile sigma(r) at radii r>10 kpc in
the Galactic stellar halo, recently derived by Battaglia et al. (2005), who
concluded that, for constant velocity anisotropy of the tracers, these data
rule out a flat circular-speed curve for the Milky Way. However, we demonstrate
that if one makes the natural assumption that the tracer density is truncated
at r_t >~ 160 kpc and falls off significantly more steeply than r^-3.5 at r >~
80 kpc, then these data are consistent with a flat circular-speed curve and a
constant velocity anisotropy comparable to that observed for halo stars in the
Solar neighbourhood. We also consider a more detailed mass model with an
exponential stellar disc and an extended non-isothermal dark-matter halo. In
this two-component model, the Milky Way's virial radius and mass are r_vir =
200 kpc and M_vir = 1.5 x 10^12 M_sun. Still assuming the tracers' velocity
anisotropy to be constant (at beta=0.5) we again find good agreement with the
observed sigma(r), so long as the tracer density is truncated near the virial
radius. These data by themselves do not allow to differentiate between
different dark-halo or total-mass models for the Milky Way, nor between
different velocity-anisotropy profiles for the tracers.Comment: 5 pages, 4 figures, accepted for publication in MNRA
Experimental Demonstration of Squeezed State Quantum Averaging
We propose and experimentally demonstrate a universal quantum averaging
process implementing the harmonic mean of quadrature variances. The harmonic
mean protocol can be used to efficiently stabilize a set of fragile squeezed
light sources with statistically fluctuating noise levels. The averaged
variances are prepared probabilistically by means of linear optical
interference and measurement induced conditioning. We verify that the
implemented harmonic mean outperforms the standard arithmetic mean strategy.
The effect of quantum averaging is experimentally tested both for uncorrelated
and partially correlated noise sources with sub-Poissonian shot noise or
super-Poissonian shot noise characteristics.Comment: 4 pages, 5 figure
Magnetohydrodynamics of Cloud Collisions in a Multi-phase Interstellar Medium
We extend previous studies of the physics of interstellar cloud collisions by
beginning investigation of the role of magnetic fields through 2D
magnetohydrodynamic (MHD) numerical simulations. We study head-on collisions
between equal mass, mildly supersonic diffuse clouds. We include a moderate
magnetic field and two limiting field geometries, with the field lines parallel
(aligned) and perpendicular (transverse) to the colliding cloud motion. We
explore both adiabatic and radiative cases, as well as symmetric and asymmetric
ones. We also compute collisions between clouds evolved through prior motion in
the intercloud medium and compare with unevolved cases.
We find that: In the (i) aligned case, adiabatic collisions, like their HD
counterparts, are very disruptive, independent of the cloud symmetry. However,
when radiative processes are taken into account, partial coalescence takes
place even in the asymmetric case, unlike the HD calculations. In the (ii)
transverse case, collisions between initially adjacent unevolved clouds are
almost unaffected by magnetic fields. However, the interaction with the
magnetized intercloud gas during the pre-collision evolution produces a region
of very high magnetic energy in front of the cloud. In collisions between
evolved clouds with transverse field geometry, this region acts like a
``bumper'', preventing direct contact between the clouds, and eventually
reverses their motion. The ``elasticity'', defined as the ratio of the final to
the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we
considered. This behavior is found both in adiabatic and radiative cases.Comment: 40 pages in AAS LaTeX v4.0, 13 figures (in degraded jpeg format).
Full resolution images as well as mpeg animations are available at
http://www.msi.umn.edu:80/Projects/twj/mhd-cc/ . Accepted for publication in
The Astrophysical Journa
Is protostellar heating sufficient to halt fragmentation? A case study of the massive protocluster G8.68-0.37
If star formation proceeds by thermal fragmentation and the subsequent
gravitational collapse of the individual fragments, how is it possible to form
fragments massive enough for O and B stars in a typical star-forming molecular
cloud where the Jeans mass is about 1Msun at the typical densities (10^4 cm^-3)
and temperatures (10K)? We test the hypothesis that a first generation of
low-mass stars may heat the gas enough that subsequent thermal fragmentation
results in fragments >=10Msun, sufficient to form B stars. We combine ATCA and
SMA observations of the massive star-forming region G8.68-0.37 with radiative
transfer modeling to derive the present-day conditions in the region and use
this to infer the conditions in the past, at the time of core formation.
Assuming the current mass/separation of the observed cores equals the
fragmentation Jeans mass/length and the region's average density has not
changed, requires the gas temperature to have been 100K at the time of
fragmentation. The postulated first-generation of low-mass stars would still be
around today, but the number required to heat the cloud exceeds the limits
imposed by the observations. Several lines of evidence suggest the observed
cores in the region should eventually form O stars yet none have sufficient raw
material. Even if feedback may have suppressed fragmentation, it was not
sufficient to halt it to this extent. To develop into O stars, the cores must
obtain additional mass from outside their observationally defined boundaries.
The observations suggest they are currently fed via infall from the very
massive reservoir (~1500Msun) of gas in the larger pc scale cloud around the
star-forming cores. This suggests that massive stars do not form in the
collapse of individual massive fragments, but rather in smaller fragments that
themselves continue to gain mass by accretion from larger scales.Comment: 23 pages, 14 figures. Accepted for publication in Ap
The LOFT Ground Segment
LOFT, the Large Observatory For X-ray Timing, was one of the ESA M3 mission
candidates that completed their assessment phase at the end of 2013. LOFT is
equipped with two instruments, the Large Area Detector (LAD) and the Wide Field
Monitor (WFM). The LAD performs pointed observations of several targets per
orbit (~90 minutes), providing roughly ~80 GB of proprietary data per day (the
proprietary period will be 12 months). The WFM continuously monitors about 1/3
of the sky at a time and provides data for about ~100 sources a day, resulting
in a total of ~20 GB of additional telemetry. The LOFT Burst alert System
additionally identifies on-board bright impulsive events (e.g., Gamma-ray
Bursts, GRBs) and broadcasts the corresponding position and trigger time to the
ground using a dedicated system of ~15 VHF receivers. All WFM data are planned
to be made public immediately. In this contribution we summarize the planned
organization of the LOFT ground segment (GS), as established in the mission
Yellow Book 1 . We describe the expected GS contributions from ESA and the LOFT
consortium. A review is provided of the planned LOFT data products and the
details of the data flow, archiving and distribution. Despite LOFT was not
selected for launch within the M3 call, its long assessment phase (> 2 years)
led to a very solid mission design and an efficient planning of its ground
operations.Comment: Proc. SPIE 9144, Space Telescopes and Instrumentation 2014:
Ultraviolet to Gamma Ray, 91446
Magellanic Cloud Structure from Near-IR Surveys II: Star Count Maps and the Intrinsic Elongation of the LMC
I construct a near-IR star count map of the LMC and demonstrate, using the
viewing angles derived in Paper I, that the LMC is intrinsically elongated. I
argue that this is due to the tidal force from the Milky Way. The near-IR data
from the 2MASS and DENIS surveys are used to create a star count map of RGB and
AGB stars, which is interpreted through ellipse fitting. The radial number
density profile is approximately exponential with a scale-length 1.3-1.5 kpc.
However, there is an excess density at large radii that may be due to the tidal
effect of the Milky Way. The position angle and ellipticity profile converge to
PA_maj = 189.3 +/- 1.4 degrees and epsilon = 0.199 +/- 0.008 for r > 5 deg. At
large radii there is a drift of the center of the star count contours towards
the near side of the plane, which can be undrestood as due to viewing
perspective. The fact that PA_maj differes from the line of nodes position
angle Theta = 122.5 +/- 8.3 (cf. Paper I) indicates that the LMC disk is not
circular, but has an intrinsic ellipticity of 0.31. The LMC is elongated in the
general direction of the Galactic center, and is elongated perpendicular to the
Magellanic Stream and the velocity vector of the LMC center of mass. This
suggests that the elongation of the LMC has been induced by the tidal force of
the Milky Way. The position angle of the line of nodes differs from the
position angle Theta_max of the line of maximum line of sight velocity
gradient: Theta_max - Theta = 20-60 degrees. This could be due to: (a)
streaming along non-circular orbits in the elongated disk; (b) uncertainties in
the transverse motion of the LMC center of mass; (c) precession and nutation of
the LMC disk as it orbits the Milky Way (expected on theoretical grounds).
[Abridged]Comment: Astronomical Journal, in press. 34 pages, LaTeX, with 7 PostScript
figures. Contains minor revisions with respect to previously posted version.
Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21
degree) stellar number-density image of the LMC constructed from RGB and AGB
stars in the 2MASS and DENIS surveys. The paper is available with higher
resolution color figures from
http://www.stsci.edu/~marel/abstracts/abs_R32.htm
A 20 GHz bright sample for {\delta} > +72{\deg}: I. Catalogue
During 2010-2011, the Medicina 32-m dish hosted the 7-feed 18-26.5 GHz
receiver built for the Sardinia Radio Telescope, with the goal to perform its
commissioning. This opportunity was exploited to carry out a pilot survey at 20
GHz over the area for {\delta} > + 72.3{\deg}. This paper describes all the
phases of the observations, as they were performed using new hardware and
software facilities. The map-making and source extraction procedures are
illustrated. A customised data reduction tool was used during the follow-up
phase, which produced a list of 73 confirmed sources down to a flux density of
115 mJy. The resulting catalogue, here presented, is complete above 200 mJy.
Source counts are in agreement with those provided by the AT20G survey. This
pilot activity paves the way to a larger project, the K-band Northern Wide
Survey (KNoWS), whose final aim is to survey the whole Northern Hemisphere down
to a flux limit of 50 mJy (5{\sigma}).Comment: 10 pages, 10 figures. Accepted by MNRA
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