Posterior Average Effects

Economists are often interested in estimating averages with respect to distributions of unobservables, such as moments of individual fixed-effects, or average partial effects in discrete choice models. For such quantities, we propose and study posterior average effects (PAE), where the average is computed conditional on the sample, in the spirit of empirical Bayes and shrinkage methods. While the usefulness of shrinkage for prediction is well-understood, a justification of posterior conditioning to estimate population averages is currently lacking. We show that PAE have minimum worst-case specification error under various forms of misspecification of the parametric distribution of unobservables. In addition, we introduce a measure of informativeness of the posterior conditioning, which quantifies the worst-case specification error of PAE relative to parametric model-based estimators. As illustrations, we report PAE estimates of distributions of neighborhood effects in the U.S., and of permanent and transitory components in a model of income dynamics

Analysis of Scramjet Flight Trajectories with Oxygen Enrichment

Scramjets are proposed as a second stage for a multi-stage access-to-space system. At present the upper limit of scramjet operation is expected to be Mach 12-14. Use of oxygen enrichment is a possible method for increasing the speed and altitude of scramjet operation. This paper involves mission analysis of scramjets using oxygen enrichment. It follows on from Smart & Tetlow [5], in which trajectory studies of a threestage rocket-scramjet-rocket access-to-space system were conducted. These calculations indicated that the net thrust (scramjet thrust - vehicle drag) of a hypersonic vehicle with three scramjet engine modules was reduced to very low levels above Mach 12. The current work examines the use of oxygen enrichment in the scramjet to increase net thrust above Mach 10. Results of the study indicate that an important effect of oxygen enrichment is to allow scramjet powered vehicle operation at higher altitude

A discontinuity in the low-mass initial mass function

The origin of brown dwarfs (BDs) is still an unsolved mystery. While the standard model describes the formation of BDs and stars in a similar way recent data on the multiplicity properties of stars and BDs show them to have different binary distribution functions. Here we show that proper treatment of these uncovers a discontinuity of the multiplicity-corrected mass distribution in the very-low-mass star (VLMS) and BD mass regime. A continuous IMF can be discarded with extremely high confidence. This suggests that VLMSs and BDs on the one hand, and stars on the other, are two correlated but disjoint populations with different dynamical histories. The analysis presented here suggests that about one BD forms per five stars and that the BD-star binary fraction is about 2%-3% among stellar systems.Comment: 14 pages, 11 figures, uses emulateapj.cls. Minor corrections and 1 reference added after being accepted by the Ap

The Variation of Integrated Star IMFs among Galaxies

The integrated galaxial initial mass function (IGIMF) is the relevant distribution function containing the information on the distribution of stellar remnants, the number of supernovae and the chemical enrichment history of a galaxy. Since most stars form in embedded star clusters with different masses the IGIMF becomes an integral of the assumed (universal or invariant) stellar IMF over the embedded star-cluster mass function (ECMF). For a range of reasonable assumptions about the IMF and the ECMF we find the IGIMF to be steeper (containing fewer massive stars per star) than the stellar IMF, but below a few Msol it is invariant and identical to the stellar IMF for all galaxies. However, the steepening sensitively depends on the form of the ECMF in the low-mass regime. Furthermore, observations indicate a relation between the star formation rate of a galaxy and the most massive young stellar cluster in it. The assumption that this cluster mass marks the upper end of a young-cluster mass function leads to a connection of the star formation rate and the slope of the IGIMF above a few Msol. The IGIMF varies with the star formation history of a galaxy. Notably, large variations of the IGIMF are evident for dE, dIrr and LSB galaxies with a small to modest stellar mass. We find that for any galaxy the number of supernovae per star (NSNS) is suppressed relative to that expected for a Salpeter IMF. Dwarf galaxies have a smaller NSNS compared to massive galaxies. For dwarf galaxies the NSNS varies substantially depending on the galaxy assembly history and the assumptions made about the low-mass end of the ECMF. The findings presented here may be of some consequence for the cosmological evolution of the number of supernovae per low-mass star and the chemical enrichment of galaxies of different mass.Comment: 27 pages, accepted for publication by Ap

The Influence of postpartum posttraumatic Stress Disorders on the childhood Development: a population-based Two-year follow-up Study

Background. Against the background of very limited evidence, the present study aimed to prospectively examine the impact of maternal postpartum PTSD symptoms on four important areas of child development, i.e. gross motor, fine motor, communication, and social-emotional development. Methods. This study is part of the large, population based Akershus Birth Cohort. Data from the hospital’s birth record as well as questionnaire data from 8 weeks and 2 years postpartum were used (n=1,472). The domains of child development that were significantly correlated with PTSD symptoms were entered into regression analyses. Interaction analyses were run to test whether the influence of postpartum PTSD symptoms on child development was moderated by child sex or infant temperament. Results. Postpartum PTSD symptoms had a prospective relationship with poor child social-emotional development two years later. This relationship remained significant even when adjusting for confounders such as maternal depression and anxiety or infant temperament. Both child sex and infant temperament moderated the association between maternal PTSD symptoms and child socialemotional development, i.e. with increasing maternal PTSD symptom load, boys and children with a difficult temperament showed to have comparatively higher levels of social-emotional problems. Conclusions. Examining four different domains of child development, we found a prospective impact of postpartum PTSD symptoms on children’s social-emotional development at two years of age. Our findings suggest that both boys and children with an early difficult temperament may be particularly susceptible to the adverse impact of postpartum PTSD symptoms. Additional studies are needed to further investigate the mechanisms at work

Chemo-Archaeological Downsizing in a Hierarchical Universe: Impact of a Top Heavy IGIMF

We make use of a semi-analytical model of galaxy formation to investigate the origin of the observed correlation between [a/Fe] abundance ratios and stellar mass in elliptical galaxies. We implement a new galaxy-wide stellar initial mass function (Top Heavy Integrated Galaxy Initial Mass Function, TH-IGIMF) in the semi-analytic model SAG and evaluate its impact on the chemical evolution of galaxies. The SFR-dependence of the slope of the TH-IGIMF is found to be key to reproducing the correct [a/Fe]-stellar mass relation. Massive galaxies reach higher [a/Fe] abundance ratios because they are characterized by more top-heavy IMFs as a result of their higher SFR. As a consequence of our analysis, the value of the minimum embedded star cluster mass and of the slope of the embedded cluster mass function, which are free parameters involved in the TH-IGIMF theory, are found to be as low as 5 solar masses and 2, respectively. A mild downsizing trend is present for galaxies generated assuming either a universal IMF or a variable TH-IGIMF. We find that, regardless of galaxy mass, older galaxies (with formation redshifts > 2) are formed in shorter time-scales (< 2 Gyr), thus achieving larger [a/Fe] values. Hence, the time-scale of galaxy formation alone cannot explain the slope of the [a/Fe]-galaxy mass relation, but is responsible for the big dispersion of [a/Fe] abundance ratios at fixed stellar mass.We further test the hyphothesis of a TH-IGIMF in elliptical galaxies by looking into mass-to-light ratios, and luminosity functions. Models with a TH-IGIMF are also favoured by these constraints. In particular, mass-to-light ratios agree with observed values for massive galaxies while being overpredicted for less massive ones; this overprediction is present regardless of the IMF considered.Comment: 24 pages, 15 figures, 2 tables. (Comments most welcome). Summited to MNRA

The influence of postpartum PTSD on breastfeeding: A longitudinal population-based study.

BACKGROUND: In most Western countries, breastfeeding rates are lower than what is recommended by the World Health Organization. Depression has been shown to influence breastfeeding outcomes; however, there is very little research on the role of postpartum posttraumatic stress disorder (PTSD). This study examined to what extent maternal postpartum PTSD predicted breastfeeding initiation, exclusive breastfeeding during the first 6 months, and continuation up to 1 and 2 years. METHODS: The study is part of the large, population-based Akershus Birth Cohort. Data from the hospital's birth record and questionnaire data from 8 weeks and 2 years postpartum were used (n = 1480). All breastfeeding variables significantly correlated with postpartum PTSD were entered into stepwise logistic regression analyses. RESULTS: Although most mothers (97.1%) initiated breastfeeding, considerably fewer adhered to the World Health Organization's breastfeeding guidelines about exclusive breastfeeding during the first 6 months (13.4%) or continued breastfeeding for 12 or 24 months postpartum (37.7% and 4.2%, respectively). Even after adjustment for important confounding variables, maternal postpartum PTSD was significantly associated with not initiating breastfeeding (aOR 5.98 [95% CI 1.79-19.97]). Postpartum PTSD was also significantly related to not continuing breastfeeding up to 12 months, although this association did not hold after adjusting for confounding variables. CONCLUSION: Identifying women at risk of not initiating breastfeeding is crucial to prevent a negative influence on infant development and the development of the mother-infant bond. Early screening and treatment of women at risk of developing postpartum PTSD might be a way forward

Complications After Systematic, Random and Image-guided Prostate Biopsy

CONTEXT: Prostate biopsy (PB) represents the gold standard method to confirm the presence of cancer. In addition to traditional random or systematic approaches, a magnetic resonance imaging (MRI)-guided technique has been introduced recently. OBJECTIVE: To perform a systematic review of complications after transrectal ultrasound (TRUS)-guided, transperineal, and MRI-guided PB. EVIDENCE ACQUISITION: We performed a systematic literature search of Web of Science, Embase, and Scopus databases up to October 2015, according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. Complications and mortality following random, systematic, and image-guided PBs were reviewed. Eighty-five references were included. EVIDENCE SYNTHESIS: The most frequent complication after PB was minor and self-limiting bleeding (hematuria and hematospermia), regardless of the biopsy approach. Occurrence of rectal bleeding was comparable for traditional TRUS-guided and image-guided PBs. Almost 25% of patients experienced lower urinary tract symptoms, but only a few had urinary retention, with higher rates after a transperineal approach. Temporary erectile dysfunction was not negligible, with a return to baseline after 1-6 mo. The incidence of infective complications is increasing, with higher rates among men with medical comorbidities and older age. Transperineal and in-bore MRI-targeted biopsy may reduce the risk of severe infectious complications. Mortality after PB is uncommon, regardless of biopsy technique. CONCLUSIONS: Complications after PB are frequent but often self-limiting. The incidence of hospitalization due to severe infections is continuously increasing. The patient's general health status, risk factors, and likelihood of antimicrobial resistance should be carefully appraised before scheduling a PB. PATIENT SUMMARY: We reviewed the variety and incidence of complications after prostate biopsy. Even if frequent, complications seldom represent a problem for the patient. The most troublesome complications are infections. To minimize this risk, the patient's medical condition should be carefully evaluated before biopsy

The scale-free character of the cluster mass function and the universality of the stellar IMF

Our recent determination of a Salpeter slope for the IMF in the field of 30 Doradus (Selman and Melnick 2005) appears to be in conflict with simple probabilistic counting arguments advanced in the past to support observational claims of a steeper IMF in the LMC field. In this paper we re-examine these arguments and show by explicit construction that, contrary to these claims, the field IMF is expected to be exactly the same as the stellar IMF of the clusters out of which the field was presumably formed. We show that the current data on the mass distribution of clusters themselves is in excellent agreement with our model, and is consistent with a single spectrum {\it by number of stars} of the type $n^\beta$ with beta between -1.8 and -2.2 down to the smallest clusters without any preferred mass scale for cluster formation. We also use the random sampling model to estimate the statistics of the maximal mass star in clusters, and confirm the discrepancy with observations found by Weidner and Kroupa (2006). We argue that rather than signaling the violation of the random sampling model these observations reflect the gravitationally unstable nature of systems with one very large mass star. We stress the importance of the random sampling model as a \emph{null hypothesis} whose violation would signal the presence of interesting physics.Comment: 9 pages emulateap