Sample Selection in Models of Academic Performance

This article shows how admission and enrollment processes affect the interpretation of simple validation studies of academic performance. In a competitive market for students, optimal behavior of admissions committees and applicants drives the simple correlation between test scores and performance toward zero, regardless of the relationship in the population of prospective students. Data from our university’s MBA program support the prediction that applicants exhibit a higher correlation between test scores and undergraduate GPAs than do current students. This suggests that standard validation studies will understate the importance of GMAT scores in predicting performance of potential MBA students

Identification by Disaggregation

Standard economic theory predicts that the actions of individual participants in competitive markets have negligible effects on market-determined aggregates. Applied researchers, and even some econometric textbooks, incorrectly infer from this that market prices can be modeled as econometrically exogenous with respect to the quantity demanded of an individual consumer. This faulty inference has even led some researchers (for example, Robert Engle, 1978; Nicholas Kiefer, 1984; Roger Waud, 1974) to employ an estimation strategy we call identification by disaggregation (IBD). This procedure attempts to circumvent the simultaneity problem in a macro regression by disaggregating the dependent variable and estimating the relationship for individual agents or sectors. This note provides a simple proof that estimates using disaggregated dependent variables suffer, on average, from the same degree of simultaneity bias as the estimates using aggregate data

Is West African Monetary Zone (WAMZ) a common currency area?

In this paper, we test whether the West African Monetary Zone (WAMZ) is a common currency area by using a structural vector autoregressive model to study the variance decomposition, impulse responses of key economic variables and linear dependence of the underlying structural shocks of the countries in the zone. The variance decomposition shows that the zone as a whole does not have common sources of shock, which is expected because of the diverse economic structures of these countries. The correlation of the structural shocks also shows that these countries respond asymmetrically to common supply, demand and monetary shocks and will therefore respond differently to a common monetary policy.It is therefore not in the interest of the individual countries to go into a monetary union now or in the near future unless the economies of these countries converge further

Assessment of benefit of advanced inflammatory bowel disease training: Challenges and solutions

BACKGROUND: Advanced inflammatory bowel disease (IBD) fellowships are available for gastroenterologists who wish to increase their expertise in complex IBD. However, little is known about the outcomes of such training. The aims of this study were to assess clinical and academic outcomes following advanced training in IBD. METHODS: We surveyed gastroenterologists who completed advanced IBD fellowships and compared competency and outcomes to gastroenterologists focusing in IBD who completed gastroenterology training alone. Participants completed a survey via REDCap. Continuous variables were compared using the Wilcoxon rank-sum test. Categorical variables were compared using chi-square or Fisher\u27s exact tests. RESULTS: A total of 104 physicians participated in the study. IBD fellowships were completed by 31 physicians (30%), of whom 29 (94%) felt their training was excellent. Management of complicated IBD (84%), research mentoring (74%), and career mentoring (71%) were felt to contribute most highly to professional development. Compared to non-advanced trained physicians, advanced trained physicians expressed higher levels of comfort with management of IBD during pregnancy ( CONCLUSIONS: This study suggests there may be clinical benefit to advanced IBD training. Importantly, this study identified that there are also unique challenges to the assessment of clinical competency in IBD training. Efforts by the IBD community to establish a registry of advanced trainees and improve competency assessments are needed

Deliquescence Behavior of Organic/Ammonium Sulfate Aerosol

Recent studies have shown that tropospheric aerosols composed of internal mixtures of organics with sulfates are quite common with the organic composing up to 50% of the particle mass. The influences of the organics on the chemical and physical properties of the aerosol are not known. In this paper, we report the solubility of a series of dicarboxylic acids in saturated ammonium sulfate solution as a function of temperature. We also report the deliquescence relative humidity (DRH) of the pure dicarboxylic acids and of mixtures of dicarboxylic acids with ammonium sulfate. For the systems studied, we find that the presence of water-soluble dicarboxylic acids caused deliquescence to occur at a lower relative humidity (RH) than pure ammonium sulfate. In contrast, the less soluble dicarboxylic acids had no measurable effect on the deliquescence relative humidity of ammonium sulfate

Radii of 88 M subdwarfs and updated radius relations for low-metallicity M-dwarf stars

M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (T_ eff) and we measure bolometric luminosities (L_ bol) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the T_ eff and L_ bol using the Stefan–Boltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to [Fe/H] of −2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.Published versio

Phase Changes in Internally Mixed Maleic Acid/Ammonium Sulfate Aerosols

A temperature controlled flow tube system equipped with Fourier transform infrared (FTIR) detection of particle phase and relative humidity was used to measure the deliquescence and efflorescence of ammonium sulfate, maleic acid, and internally mixed maleic acid/ammonium sulfate particles. Our results indicate that maleic acid aerosols begin to take up water starting at a low relative humidity, ∼20%, and continue the constant uptake of water until the final deliquescence relative humidity (DRH), 89%, is reached. Internally mixed particles containing maleic acid and ammonium sulfate were found to deliquesce at a lower relative humidity (RH) than either of the pure species. Efflorescence studies indicated that while pure maleic acid particles crystallize at ∼18% RH, pure ammonium sulfate and all mixed aerosols effloresce at or just below 30% RH. Taken together, our results suggest that the presence of water-soluble organics internally mixed with ammonium sulfate aerosol could increase the range of conditions under which the aerosol is a solution