Maximum power point tracker Patent

Power point tracker for maintaining optimal output voltage of power sourc

Missing Work and Quitting Work: Child Care-Related Employment Problems

Qualitative research points to logistical problems in coordinating child care as a key obstacle to maternal employment for low-income mothers. But quantitative research has largely overlooked this everyday aspect of combining work and family. This article provides quantitative analyses of child-care related employment problems among urban working mothers of infants and asks how social support, the complexity of work and care arrangements and demographic characteristics relate to these problems. We use the Fragile Families and Child Well-Being Study to estimate logistic regression models of child care failure and missing or quitting work due to care-related problems. Child-care related problems are widespread regardless of race, class or family structure. Mothers with potential backup providers are less likely to experience care-related problems. Mothers who hold more than one job, use more than one care provider or change providers encounter problems more often. Logistical challenges surrounding child care represent a serious obstacle to continued employment for all urban working mothers. Care-related employment problems are more closely associated with the availability of backup care and the complexity of work and care arrangements than with class. These problems merit further study given their potential impact on the gender wage gap.

The Changing Role of Auditors in Corporate Tax Planning

This paper examines changes in the role that auditors play in corporate tax planning following recent events, including the well-known accounting scandals, passage of the Sarbanes-Oxley Act, and regulatory actions by the SEC and PCAOB. On the whole, these events have increased the sensitivity to and scrutiny of auditor independence. We examine the effects of these events on the market for tax planning, in particular the longstanding link between audit and tax services. While the effects are recent, they are already being seen in the data. Specifically, there has already been a dramatic shift in the market for tax planning away from obtaining tax planning services from one's auditor. We estimate that the ratio of tax fees to audit fees paid to the auditors of firms in the S&P 500 decline from approximately one in 2001 to one-fourth in 2004. At the same time, we find no evidence of a general decline in spending for tax services. In sum, the evidence indicates a decoupling of the longstanding link between audit and tax services, such that firms are shifting their purchase of tax services away from their auditor and towards other providers.

Resolution of a paradox: Hummingbird flight at high elevation does not come without a cost

Flight at high elevation is energetically demanding because of parallel reductions in air density and oxygen availability. The hovering flight of hummingbirds is one of the most energetically expensive forms of animal locomotion, but hummingbirds are nonetheless abundant at high elevations throughout the Americas. Two mechanisms enhance aerodynamic performance in high-elevation hummingbirds: increase in wing size and wing stroke amplitude during hovering. How do these changes in morphology, kinematics, and physical properties of air combine to influence the aerodynamic power requirements of flight across elevations? Here, we present data on the flight performance of 43 Andean hummingbird species as well as a 76-taxon multilocus molecular phylogeny that served as the historical framework for comparative analyses. Along a 4,000-m elevational transect, hummingbird body mass increased systematically, placing further aerodynamic demands on high-elevation taxa. However, we found that the minimum power requirements for hovering flight remain constant with respect to elevation because hummingbirds compensate sufficiently through increases in wing size and stroke amplitude. Thus, high-elevation hummingbirds are not limited in their capacity for hovering flight despite the challenges imposed by hypobaric environments. Other flight modes including vertical ascent and fast forward flight are more mechanically and energetically demanding, and we accordingly also tested for the maximum power available to hummingbirds by using a load-lifting assay. In contrast to hovering, excess power availability decreased substantially across elevations, thereby reducing the biomechanical potential for more complex flight such as competitive and escape maneuvers

Robust Inference with Clustered Data

In this paper we survey methods to control for regression model error that is correlated within groups or clusters, but is uncorrelated across groups or clusters. Then failure to control for the clustering can lead to understatement of standard errors and overstatement of statistical significance, as emphasized most notably in empirical studies by Moulton (1990) and Bertrand, Duflo and Mullainathan (2004). We emphasize OLS estimation with statistical inference based on minimal assumptions regarding the error correlation process. Complications we consider include cluster-specific fixed effects, few clusters, multi-way clustering, more efficient feasible GLS estimation, and adaptation to nonlinear and instrumental variables estimators.Cluster robust, random eects, xed eects, dierences in dierences, cluster bootstrap, few clusters, multi-way clusters.

Robust Inference with Clustered Data

In this paper we survey methods to control for regression model error that is correlated within groups or clusters, but is uncorrelated across groups or clusters. Then failure to control for the clustering can lead to understatement of standard errors and overstatement of statistical significance, as emphasized most notably in empirical studies by Moulton (1990) and Bertrand, Duflo and Mullainathan (2004). We emphasize OLS estimation with statistical inference based on minimal assumptions regarding the error correlation process. Complications we consider include cluster-specific fixed effects, few clusters, multi-way clustering, more efficient feasible GLS estimation, and adaptation to nonlinear and instrumental variables estimators.Cluster robust, random effects, fixed effects, differences in differences, cluster bootstrap, few clusters, multi-way clusters.

Statistical Uncertainties in Temperature Diagnostics for Hot Coronal Plasma Using the ASCA SIS

Statistical uncertainties in determining the temperatures of hot (0.5 keV to 10 keV) coronal plasmas are investigated. The statistical precision of various spectral temperature diagnostics is established by analyzing synthetic ASCA Solid-state Imaging Spectrometer (SIS) CCD spectra. The diagnostics considered are the ratio of hydrogen-like to helium-like line complexes of $Z\ge14$ elements, line-free portions of the continuum, and the entire spectrum. While fits to the entire spectrum yield the highest statistical precision, it is argued that fits to the line-free continuum are less susceptible to atomic data uncertainties but lead to a modest increase in statistical uncertainty over full spectral fits. Temperatures deduced from line ratios can have similar accuracy but only over a narrow range of temperatures. Convenient estimates of statistical accuracies for the various temperature diagnostics are provided which may be used in planning ASCA SIS observations.Comment: postscript file of 8 pages+3 figures; 4 files tarred, compressed and uuencoded. To appear in the Astrophysical Journal Letters; contents copyright 1994 American Astronomical Societ