Optimal Image Reconstruction in Radio Interferometry

We introduce a method for analyzing radio interferometry data which produces maps which are optimal in the Bayesian sense of maximum posterior probability density, given certain prior assumptions. It is similar to maximum entropy techniques, but with an exact accounting of the multiplicity instead of the usual approximation involving Stirling's formula. It also incorporates an Occam factor, automatically limiting the effective amount of detail in the map to that justified by the data. We use Gibbs sampling to determine, to any desired degree of accuracy, the multi-dimensional posterior density distribution. From this we can construct a mean posterior map and other measures of the posterior density, including confidence limits on any well-defined function of the posterior map.Comment: 41 pages, 11 figures. High resolution figures 8 and 9 available at http://www.astro.uiuc.edu/~bwandelt/SuttonWandelt200

The Enrollment Effects of Merit-Based Financial Aid: Evidence from Georgia's HOPE Scholarship

This paper examines the effects of Georgia's merit-based HOPE Scholarship on college enrollment. Introduced in 1993, the HOPE Scholarship covers tuition, fees, and book expenses for students attending Georgia public colleges, and provides a subsidy of comparable value to students attending in-state private colleges, without any income restrictions. Treating HOPE as a natural experiment, we contrast college enrollment in Georgia with those in the other member states of the Southern Regional Educational Board using IPEDS data for the period 1988-97. We estimate that the HOPE increased total freshmen enrollment by 5.9 percent, with the gains concentrated in 4-year schools. For freshmen recently graduated from high school attending 4-year colleges, two-thirds of the program effect is explained by a decrease in students leaving the state. Both white and black enrollments increased because of HOPE, with the state's historically-black institutions playing an important role. Finally, the total HOPE-induced enrollment increase represents only 15 percent freshmen scholarship recipients.Higher Education, Enrollment, HOPE, Merit-based Aid

Non-cognitive Skills and the Gender Disparities in Test Scores and Teacher Assessments: Evidence from Primary School

We extend the analysis of early-emerging gender differences in academic achievement to include both (objective) test scores and (subjective) teacher assessments. Using data from the 1998-99 ECLS-K cohort, we show that the grades awarded by teachers are not aligned with test scores, with the disparities in grading exceeding those in testing outcomes and uniformly favoring girls, and that the misalignment of grades and test scores can be linked to gender differences in non-cognitive development. Girls in every racial category outperform boys on reading tests and the differences are statistically significant in every case except for black fifth-graders. Boys score at least as well on math and science tests as girls, with the strongest evidence of a gender gap appearing among whites. However, boys in all racial categories across all subject areas are not represented in grade distributions where their test scores would predict. Even those boys who perform equally as well as girls on reading, math and science tests are nevertheless graded less favorably by their teachers, but this less favorable treatment essentially vanishes when non-cognitive skills are taken into account. White boys who perform on par with white girls on these subject-area tests and exhibit the same non-cognitive skill level are graded similarly. For some specifications there is evidence of a grade "bonus" for white boys with test scores and behavior like their girl counterparts. While the evidence is a little weaker for blacks and Hispanics, the message is essentially the same.gender differences, test scores, grades, educational attainment

Student Responses to Merit Retention Rules

A common justification for HOPE-style merit-aid programs is to promote and reward academic achievement, thereby inducing greater investments in human capital. However, grade-based eligibility and retention rules encourage other behavioral responses. Using the longitudinal records of all undergraduates who enrolled at the University of Georgia (UGA) between 1989 and 1997, we estimate the effects of HOPE on course enrollment, withdrawal and completion, and the diversion of course taking from the academic year to the summer, treating non-residents as a control group. First, we find that HOPE decreased full-load enrollments and increased course withdrawals among resident freshmen. The combination of these responses results in an 11\% lower probability of full-load completion and an annual average reduction in credits completed of 1.0. The latter implies that between 1993 and 1997 Georgia-resident freshmen completed 15,710 fewer credit hours or 3,142 individual course enrollments than non-residents. Second, the scholarship's influence on course-taking behavior is concentrated on students with GPAs on or below the scholarship-retention margin. Third, the effect increased as the income cap was lifted and more students became eligible for the award. Fourth, these freshmen credit-hour reductions represent an intertemporal substitution, not a general slowdown in academic progress. Finally, residents diverted an average of 1.65 more credits from the regular academic year to the first summer term after their matriculation, which amounts to a 72\% rise in summer course taking.Education, Merit-based aid, Education Finance, HOPE Scholarship

Displacement- and Timing-Noise Free Gravitational-Wave Detection

Motivated by a recently-invented scheme of displacement-noise-free gravitational-wave detection, we demonstrate the existence of gravitational-wave detection schemes insusceptible to both displacement and timing (laser) noises, and are thus realizable by shot-noise-limited laser interferometry. This is possible due to two reasons: first, gravitational waves and displacement disturbances contribute to light propagation times in different manners; second, for an N-detector system, the number of signal channels is of the order O(N^2), while the total number of timing- and displacement-noise channels is of the order O(N).Comment: 4 pages, 3 figures; mistake correcte

Merit aid and sorting: the effects of HOPE-style scholarships on college ability stratification

In the last fifteen years there has been a significant increase in merit aid. Coincident with this increase in merit aid has been increased attention to sorting in various aspects of life, especially in education. This paper examines the extent to which merit-based aid exacerbates or ameliorates sorting by ability in higher education. We use panel data from Peterson's Guide to Colleges and the Integrated Postsecondary Education Data System (IPEDS) to evaluate this relationship. Our difference-in-differences estimates show that HOPE increased the quality of entering freshmen in Georgia institutions relative to their out-of-state peers. At the highest-quality institutions HOPE raised all measures of student quality and the homogeneity of students by ability. The lowest-quality institutions experienced no statistically significant effect from HOPE on any measure of student quality. We conclude that state-sponsored merit aid programs increased the retention of high ability students for college and also increased the ability stratification of institutions within states. We also examined two indirect measures of student selectivity-acceptance and yield rates. HOPE decreases acceptance rates at all types of institutions, but the percentage change is largest at the universities, which are most space constrained. HOPE increased yield rates for universities but not for any other institution categories. Together these results suggest that HOPE substantially increased the selectivity at universities