Showing papers by "Dartmouth College published in 1997"

Exceptional Exporter Performance: Cause, Effect, or Both?

Abstract: A growing body of empirical work has documented the superior performance characteristics" of exporting plants and firms relative to non-exporters. Employment, shipments and capital intensity are all higher at exporters at any given moment. This paper asks whether good" firms become exporters or whether exporting improves firm performance. The evidence is quite" clear on one point: good firms become exporters, both growth rates and levels of success measures" are higher ex-ante for exporters. The benefits of exporting for the firm are less clear. Employment" growth and the probability of survival are both higher for exporters; however growth is not superior, particularly over longer horizons.

Value Versus Growth: The International Evidence

Abstract: Value stocks have higher returns than growth stocks in markets around the world. For 1975-95, the difference between the average returns on global portfolios of high and low book-to-market stocks is 7.60% per year, and value stocks outperform growth stocks in 12 of 13 major markets. An international CAPM cannot explain the value premium, but a two-factor model that includes a risk factor for relative distress captures the value premium in international returns.

Legal Determinants of External Finance

Abstract: Using a sample of 49 countries, we show that countries with poorer investor protections, measured by both the character of legal rules and the quality of law enforcement, have smaller and narrower capital markets. These findings apply to both equity and debt markets. In particular, French civil law countries have both the weakest investor protections and the least developed capital markets, especially as compared to common law countries.

Adaptive greedy approximations

Abstract: The problem of optimally approximating a function with a linear expansion over a redundant dictionary of waveforms is NP-hard. The greedy matching pursuit algorithm and its orthogonalized variant produce suboptimal function expansions by iteratively choosing dictionary waveforms that best match the function’s structures. A matching pursuit provides a means of quickly computing compact, adaptive function approximations. Numerical experiments show that the approximation errors from matching pursuits initially decrease rapidly, but the asymptotic decay rate of the errors is slow. We explain this behavior by showing that matching pursuits are chaotic, ergodic maps. The statistical properties of the approximation errors of a pursuit can be obtained from the invariant measure of the pursuit. We characterize these measures using group symmetries of dictionaries and by constructing a stochastic differential equation model. We derive a notion of the coherence of a signal with respect to a dictionary from our characterization of the approximation errors of a pursuit. The dictionary elements slected during the initial iterations of a pursuit correspond to a function’s coherent structures. The tail of the expansion, on the other hand, corresponds to a noise which is characterized by the invariant measure of the pursuit map. When using a suitable dictionary, the expansion of a function into its coherent structures yields a compact approximation. We demonstrate a denoising algorithm based on coherent function expansions.

Acyl-coenzyme a:cholesterol acyltransferase

Abstract: Due to its presumed role in regulating cellular cholesterol homeostasis, and in various pathophysiological conditions, acyl-coenzyme A:cholesterol acyltransferase (ACAT) has attracted much attention. Cloning the ACAT gene provides the necessary tool to advance molecular studies of this enzyme. The topics reviewed in this chapter include the pathophysiological roles of ACAT, the biochemistry and molecular biology of the ACAT protein and the ACAT gene, and the mode of regulation by sterol or nonsterol agents in mammalian cells. In addition, we present a working model linking the presumed allosteric property of ACAT with cholesterol trafficking into and out of the endoplasmic reticulum.

Prospective study of blunt aortic injury: Multicenter trial of the American Association for the Surgery of Trauma

Abstract: Background: Blunt aortic injury is a major cause of death from blunt trauma. Evolution of diagnostic techniques and methods of operative repair have altered the management and posed new questions in recent years. Methods: This study was a prospectively conducted multicenter trial involving 50 trauma centers in North America under the direction of the Multi-institutional Trial Committee of the American Association for the Surgery of Trauma. Results: There were 274 blunt aortic injury cases studied over 2.5 years, of which 81% were caused by automobile crashes. Chest computed tomography and transesophageal echocardiography were applied in 88 and 30 cases, respectively, and were 75 and 80% diagnostic, respectively. Two hundred seven stable patients underwent planned thoracotomy and repair. Clamp and sew technique was used in 73 (35%) and bypass techniques in 134 (65%). Overall mortality was 31%, with 63% of deaths being attributable to aortic rupture; mortality was not affected by method of repair. Paraplegia occurred postoperatively in 8.7%. Logistic regression analysis demonstrated clamp and sew (p = 0.002) and aortic cross clamp time of 30 minutes (p = 0.01) to be associated with development of postoperative paraplegia. Conclusions: Rupture after hospital admission remains a major problem. Although newer diagnostic techniques are being applied, at this time aortography remains the diagnostic standard. Aortic cross clamp time beyond 30 minutes was associated with paraplegia; bypass techniques, which provide distal aortic perfusion, produced significantly lower paraplegia rates than the clamp and sew approach.

A novel polymorphism of FcgammaRIIIa (CD16) alters receptor function and predisposes to autoimmune disease.

Abstract: A novel polymorphism in the extracellular domain 2 (EC2) of FcgammaRIIIA affects ligand binding by natural killer (NK) cells and monocytes from genotyped homozygous normal donors independently of receptor expression. The nonconservative T to G substitution at nucleotide 559 predicts a change of phenylalanine (F) to valine (V) at amino acid position 176. Compared with F/F homozygotes, FcgammaRIIIa expressed on NK cells and monocytes in V/V homozygotes bound more IgG1 and IgG3 despite identical levels of receptor expression. In response to a standard aggregated human IgG stimulus, FcgammaRIIIa engagement on NK cells from V/V (high-binding) homozygotes led to a larger rise in [Ca2+]i, a greater level of NK cell activation, and a more rapid induction of activation-induced cell death (by apoptosis). Investigation of an independently phenotyped normal cohort revealed that all donors with a low binding phenotype are F/F homozygotes, while all phenotypic high binding donors have at least one V allele. Initial analysis of 200 patients with SLE indicates a strong association of the low binding phenotype with disease, especially in patients with nephritis who have an underrepresentation of the homozygous high binding phenotype. Thus, the FcgammaRIIIa polymorphism at residue 176 appears to impact directly on human biology, an effect which may extend beyond autoimmune disease characterized by immune complexes to host defense mechanisms.

Open‐top designs for manipulating field temperature in high‐latitude ecosystems

Abstract: Passive open-top devices have been proposed as a method to experimentally increase temperature in high-latitude ecosystems. There is, however, little documentation on the efficacy of these devices. This paper examines the performance of four open-top chambers for altering temperature at six sites in the Arctic and Antarctica. Most of the heating effect was due to daytime warming above ambient; occasional night-time cooling below ambient, especially of air temperatures, depressed mean daily temperature. The mean daily temperatures at four arctic sites were generally increased by 1.2–1.8 °C; but occasionally, temperature depressions also occurred. Under optimal conditions at the antarctic site (dry soils, no vegetation, high radiation) mean daily soil temperatures were increased by +2.2 °C (–10 cm) to +5.2 °C (0 cm). Protection from wind may play a more important role than temperature per se in providing a favourable environment for plant growth within open-top devices. Wind speed had a generally negative impact on mean daily temperature. Daily global radiation was both positively and negatively related to chamber temperature response. The effect of chambers on snow accumulation was variable with the Alexandra Fjord site showing an increased accumulation in chambers but no difference in the date of snowmelt, while at Latnjajaure in a deep snowfall site, snowmelt occurred 1–2 weeks earlier in chambers, potentially increasing the growing season. Selection of a passive temperature-enhancing system requires balancing the temperature enhancement desired against potential unwanted ecological effects such as chamber overheating and altered light, moisture, and wind. In general, the more closed the temperature-enhancing system, the higher is the temperature enhancement, but the larger are the unwanted ecological effects. Open-top chambers alter temperature significantly and minimize most unwanted ecological effects; as a consequence, these chambers are a useful tool for studying the response of high-latitude ecosystems to warming.

The Recent Decline in Mortality From Coronary Heart Disease, 1980-1990: The Effect of Secular Trends in Risk Factors and Treatment

Abstract: Objective. —To examine whether secular trends in risk factor levels and improvements in treatment can account for the observed decline in coronary heart disease mortality in the United States from 1980 to 1990 and to analyze the proportional contribution of these changes. Data Sources. —Literature review, US statistics, health surveys, and ongoing clinical trials. Study Selection. —Data representative of the US situation nationwide reported in adequate detail. Data Extraction. —A computer-simulation state-transition model of the US population between the ages of 35 and 84 years was developed to forecast coronary mortality. The input variables were estimated such that the combination of values led to an adequate agreement with reported coronary mortality figures. Subsequently, secular trends were modeled. Data Synthesis. —Actual coronary mortality in 1990 was 34% (127 000 deaths) lower than would be predicted if risk factor levels, case-fatality rates, and event rates in those with and without coronary disease remained the same as in 1980. When secular changes in these factors were included in the model, predicted coronary mortality in 1990 was within 3% (10 000 deaths) of the observed mortality and explained 92% of the decline; only 25% of the decline was explained by primary prevention, while 29% was explained by secondary reduction in risk factors in patients with coronary disease and 43% by other improvements in treatment in patients with coronary disease. Conclusions. —These results suggest that primary and secondary risk factor reductions explain about 50% of the striking decline in coronary mortality in the United States between 1980 and 1990 but that more than 70% of the overall decline in mortality has occurred among patients with coronary disease.

Optimal choice for number of strands in a litz-wire transformer winding

Abstract: The number and diameter of strands to minimize loss in a litz-wire transformer winding is determined. With fine stranding, the AC resistance factor can be decreased, but DC resistance increases as a result of the space occupied by insulation. A power law to model insulation thickness is combined with standard analysis of proximity-effect losses to find the optimal stranding. Suboptimal choices under other constraints are also determined.

Epidemiology of sepsis syndrome in 8 academic medical centers.

Abstract: Context. —Sepsis syndrome is a leading cause of mortality in hospitalized patients. However, few studies have described the epidemiology of sepsis syndrome in a hospitalwide population. Objective. —To describe the epidemiology of sepsis syndrome in the tertiary care hospital setting. Design. —Prospective, multi-institutional, observational study including 5-month follow-up. Setting. —Eight academic tertiary care centers. Methods. —Each center monitored a weighted random sample of intensive care unit (ICU) patients, non-ICU patients who had blood cultures drawn, and all patients who received a novel therapeutic agent or who died in an emergency department or ICU. Sepsis syndrome was defined as the presence of either a positive blood culture or the combination of fever, tachypnea, tachycardia, clinically suspected infection, and any 1 of 7 confirmatory criteria. Estimates of total cases expected annually were extrapolated from the number of cases, the period of observation, and the sampling fraction. Results. —From January 4, 1993, to April 2, 1994, 12759 patients were monitored and 1342 episodes of sepsis syndrome were documented. The extrapolated, weighted estimate of hospitalwide incidence (mean±95% confidence limit) of sepsis syndrome was 2.0±0.16 cases per 100 admissions, or 2.8±0.17 per 1000 patient-days. The unadjusted attack rate for sepis syndrome between individual centers differed by as much as 3-fold, but after adjustment for institutional differences in organ transplant populations, variation from the expected number of cases was reduced to 2-fold and was not statistically significant overall. Patients in ICUs accounted for 59% of total extrapolated cases, non-ICU patients with positive blood cultures for 11%, and non-ICU patients with negative blood cultures for 30%. Septic shock was present at onset of sepsis syndrome in 25% of patients. Bloodstream infection was documented in 28%, with gram-positive organisms being the most frequent isolates. Mortality was 34% at 28 days and 45% at 5 months. Conclusions. —Sepsis syndrome is common in academic hospitals, although the overall rates vary considerably with the patient population. A substantial fraction of cases occur outside ICUs. An understanding of the hospitalwide epidemiology of sepsis syndrome is vital for rational planning and treatment of hospitalized patients with sepsis syndrome, especially as new and expensive therapeutic agents become available.

Primary bone tumors of the spine. Terminology and surgical staging

Abstract: Appropriate application of an oncologic staging system is required to evaluate the relationship among histologic types, management, and outcome of primary bone tumors. A commonly accepted terminology for surgical procedures and for definition of tumor extent is needed for surgical planning and clinical reviews. The principles of the Enneking system for classifying stages of tumors are emphasized and applied to the spine using a practical approach for surgical staging.

The Reasons for Wage Rigidity: Evidence From a Survey of Firms

Abstract: A survey of 184 firms was conducted to mvestIgate the reasons for wage ngid­ ity The strongest support was found for explanations based on adverse selection in quits and on the effect of wages on effort. In addition, survey respondents mdi­ cated that reducmg turnover IS an Important explanation of wage ngIdIty for wrute-collar workers, and that implicIt contracts are an important explanation for other workers. Respondents also belIeved that effort responds more strongly to wage decreases than to wage mcreases and that wage decreases have a greater Impact on the effort of low-skIlled workers than of hIgh-skIlled workers.

Neurospora wc-1 and wc-2: Transcription, Photoresponses, and the Origins of Circadian Rhythmicity

Abstract: Circadian rhythmicity is universally associated with the ability to perceive light, and the oscillators (“clocks”) giving rise to these rhythms, which are feedback loops based on transcription and translation, are reset by light. Although such loops must contain elements of positive and negative regulation, the clock genes analyzed to date—frq in Neurospora andper and tim in Drosophila—are associated only with negative feedback and their biochemical functions are largely inferred. The white collar–1 and white collar–2 genes, both global regulators of photoresponses inNeurospora, encode DNA binding proteins that contain PAS domains and are believed to act as transcriptional activators. Data shown here suggest that wc-1 is a clock-associated gene andwc-2 is a clock component; both play essential roles in the assembly or operation of the Neurospora circadian oscillator. Thus DNA binding and transcriptional activation can now be associated with a clock gene that may provide a positive element in the feedback loop. In addition, similarities between the PAS-domain regions of molecules involved in light perception and circadian rhythmicity in several organisms suggest an evolutionary link between ancient photoreceptor proteins and more modern proteins required for circadian oscillation.

What Accounts for the Variation in Retirement Wealth Among U.S. Households

Abstract: Household survey data consistently depict large variations in saving and wealth, even among households with similar socio-economic characteristics. Within the context of the life cycle hypothesis, families with identical lifetime resources might choose to accumulate different levels of wealth for a variety of reasons, including variation in time preference rates, risk tolerance, exposure to uncertainty, relative tastes for work and leisure at advanced ages, income replacement rates, and so forth. These factors can be divided into a small number of classes, each with a distinctive implication concerning the relation between accumulated wealth and the shape of the consumption profile. By examining this relation empirically, one can test for the presence or absence of particular factors. Using the Panel Study of Income Dynamics and the Consumer Expenditure Survey, we find very little support for life cycle models that rely on the above factors to explain wealth variation. The data are, however, consistent with "rule of thumb" or "mental accounting" theories of wealth accumulation.

Homotypic vacuolar fusion mediated by t- and v-SNAREs

Abstract: Membrane fusion is necessary both in the eukaryotic secretory pathway and for the inheritance of organelles during the cell cycle. In the secretory pathway, heterotypic fusion takes place between small transport vesicles and organelles. It requires N-ethylmaleimide-sensitive fusion protein (NSF/Sec18p), soluble NSF attachment proteins (SNAPs/Sec17p) and SNAP receptors (SNAREs). SNAREs are integral membrane proteins (v-SNAREs on vesicles, t-SNAREs on the target organelles) and are thought to provide specificity to the fusion process1–5. It has been suggested that Sec17p and SeclSp bind to v-SNARE/t-SNARE complexes and mediate the membrane fusion event1–3. Homotypic fusion of yeast vacuoles also requires Sec17p and Sec18p (ref. 6), but in vitro they are needed only to 'prime' the vacuoles, not for subsequent docking or fusion7,8. It has been unclear whether these reactions involve SNAREs that are similar to those previously identified in heterotypic fusion systems and, hence, whether the actions of Sec18p/NSF and Sec17p/αSNAP in these systems can be compared. Here we identify typical v- and t-SNAREs on the yeast vacuolar membrane. Although both are normally present, vacuoles containing only the v-SNARE can fuse with those containing only the t-SNARE. Vacuoles containing neither SNARE cannot fuse with those containing both, demonstrating that docking is mediated by cognate SNAREs on the two organelle membranes. Even when t- and v-SNAREs are on separate membranes, Sec17p and Sec18p act at the priming stage. Their action is not required at the point of assembly of the SNARE complex, nor for the fusion event itself.

The use of isotope tracers for identifying populations of migratory birds

Abstract: To determine whether stable isotopes can be used for identifying the geographic origins of migratory bird populations, we examined the isotopic composition of hydrogen (deuterium, δD), carbon (δ13C), and strontium (δ87Sr) in tissues of a migratory passerine, the black-throated blue warbler (Dendroica caerulescens), throughout its breeding range in eastern North America. δD and δ13C values in feathers, which are grown in the breeding area, varied systematically along a latitudinal gradient, being highest in samples from the southern end of the species’ breeding range in Georgia and lowest in southern Canada. In addition, δD decreased from east to west across the northern part of the breeding range, from New Brunswick to Michigan. δ87Sr ratios were highest in the Appalachian Mountains, and decreased towards the west. These patterns are consistent with geographical variation in the isotopic composition of the natural environment, i.e., with that of precipitation, plants, and soils for δD, δ13C, and δ87Sr, respectively. Preliminary analyses of the δD and δ13C composition of feathers collected from warblers in their Caribbean winter grounds indicate that these individuals were mostly from northern breeding populations. Furthermore, variances in isotope ratios in samples from local areas in winter tended to be larger than those in summer, suggesting that individuals from different breeding localities may mix in winter habitats. These isotope markers, therefore, have the potential for locating the breeding origins of migratory species on their winter areas, for quantifying the degree of mixing of breeding populations on migratory and wintering sites, and for documenting other aspects of the population structure migratory animals – information needed for studies of year-round ecology of these species as well as for their conservation. Combining information from several stable isotopes will help to increase the resolution for determining the geographic origins of individuals in such highly vagile populations.

The lateral pressure profile in membranes: a physical mechanism of general anesthesia.

Abstract: A mechanism of general anesthesia is suggested and investigated using lattice statistical thermodynamics. Bilayer membranes are characterized by large lateral stresses that vary with depth within the membrane. Incorporation of amphiphilic and other interfacially active solutes into the bilayer is predicted to increase the lateral pressure selectively near the aqueous interfaces, compensated by decreased lateral pressure toward the center of the bilayer. General anesthesia likely involves inhibition of the opening of the ion channel in a postsynaptic ligand-gated membrane protein. If channel opening increases the cross-sectional area of the protein more near the aqueous interface than in the middle of the bilayer, then the anesthetic-induced increase in lateral pressure near the interface will shift the protein conformational equilibrium to favor the closed state, since channel opening will require greater work against this higher pressure. This hypothesis provides a truly mechanistic and thermodynamic understanding of anesthesia, not just correlations of potency with structural or thermodynamic properties. Calculations yield qualitative agreement with anesthetic potency at clinical anesthetic membrane concentrations and predict the alkanol cutoff and anomalously low potencies of strongly hydrophobic molecules with little or no attraction for the aqueous interface, such as perfluorocarbons.

Evaluation of mental retardation: Recommendations of a consensus conference

Abstract: A Consensus Conference utilizing available literature and expert opinion sponsored by the American College of Medical Genetics in October 1995 evaluated the rational approach to the individual with mental retardation. Although no uniform protocol replaces individual clinician judgement, the consensus recommendations were as follows: 1. The individual with mental retardation, the family, and medical care providers benefit from a focused clinical and laboratory evaluation aimed at establishing causation and in providing counseling, prognosis, recurrence risks, and guidelines for management. 2. Essential elements of the evaluation include a three-generation pedigree: pre-, peri-, and post-natal history, complete physical examination focused on the presence of minor anomalies, neurologic examination, and assessment of the behavioral phenotype. 3. Selective laboratory testing should, in most patients, include a banded karyotype. Fragile X testing should be strongly considered in both males and females with unexplained mental retardation, especially in the presence of a positive family history, a consistent physical and behavioral phenotype and absence of major structural abnormalities. Metabolic testing should be initialed in the presence of suggestive clinical and physical findings. Neuroimaging should be considered in patients without a known diagnosis especially in the presence of neurologic symptoms, cranial contour abnormalities, microcephaly, or macrocephaly. In most situations MRI is the testing modality of choice. 4. Sequential evaluation of the patient, occasionally over several years, is often necessary for diagnosis, allowing for delineation of the physical and behavioral phenotype, a logical approach to ancillary testing and appropriate prognostic and reproductive counseling.

Breast Cancer Surveillance Consortium: a national mammography screening and outcomes database.

Abstract: M ammography is the primary method of detecting early stage breast cancer and has been shown in randomized clinical trials to reduce breast cancer mortality, especially among women 50 years old and older [1-5]. Authorities in cancer screening have bong recognized that the level of efficacy of screening demonstrated in randomized clinical trials may not pertain to community practice for several reasons [6]. These reasons include possible differences in the population groups receiving screening, lower accuracy of screening mammography in the community, and lower compliance with diagnostic follow-up and treatment in community practice, which may result in more adverse outcomes. Screening effectiveness in community practice today could exceed that estimated in trials because the technical and interpretative quality of mammography has improved since the trials were performed. Furthermore, clinical trial efficacy has been estimated on the basis of assignment to receive screening; to the extent that women assigned to screening were not screened or that women in the control groups were screened, efficacy in trials may have been underestimated. To optimally evaluate the performance of mammography in a community setting, the screening prevalence and patterns and the associated sensitivity, specificity, and predictive value of mammography in community screening programs should be determined by linkage with cancer outcomes [7, 8]. A program of monitoring should also provide data on specific populations, such as rural and minority subgroups, that are traditionally underserved by screening programs and that may have different breast cancer mortality rates [9]. Before the Mammography Quality Standards Act (MQSA) of 1992, most mammography facilities in the United States did not maintain records that could provide reliable and comprehensive data to evaluate the performance of screening mammography [10]. The concept of a medical audit of outcomes data had been proposed [ 1 1 ] but has not been routinely practiced in the community. The interim regulations of the MQSA mandated maintaining mammography data and performing a medical outcomes audit [12]. In practical terms, the medical audit requirement of the MQSA was limited to an analysis of patients with tests interpreted as “suspicious abnormality” or “highly suggestive for malignancy,” which permits evaluation of the positive predictive value of such interpretations. However, the MQSA does not require linkage to populationbased cancer registry data or another source of pathology data, without which it is impossible to accurately assess the outcomes of patients with mammograms interpreted as having normal findings. To understand the full effect of

Site-dependent regulation of population size:a new synthesis

Abstract: The nature and extent of population regulation remains a principal unanswered question for many types of organisms, despite extensive research. In this paper, we provide a new synthesis of theoretical and empirical evidence that elucidates and extends a mechanism of population regulation for species whose individuals preemptively use sites that differ in suitability. The sites may be territories, refuges from predation, oviposition sites, etc. The mechanism, which we call site dependence, is not an alternative to density dependence; rather, site dependence is one of several mechanisms that potentially generate the negative feedback required for regulation. Site dependence has two major features: (1) environmentally caused heterogeneity among sites in suitability for reproduction and/or survival; and (2) preemptive site occupancy, with the tendency for individuals to move to sites of higher quality as they become available. Simulation modeling shows that these two features, acting in concert, generate nega...

Impact of tillage practices on organic carbon and nitrogen storage in cool, humid soils of eastern Canada

Abstract: Soil organic matter storage capacity in agroecosystems varies with soil type, climate and agricultural management practices. The effects of different tillage systems on organic C and N storage were determined for a range of soils of eastern Canada mainly under continuous corn and small grain cereal production. Soil profiles from eight sites on which comparative tillage experiments had been performed for up to 11 years were sampled to a 60 cm depth in four increments (0–10, 10–20, 20–40 and 40–60cm). Organic C and N contents and dry bulk density were determined for each sampling depth. Bulk density measurements showed that the total soil mass in the soil profiles was not influenced by the tillage systems. No significant differences were found between tillage treatments in the total organic C and N storage down to 60 cm depth; the soil profiles under no-till (NT) and chisel plowing (CP) generally did not contain more C and N than those under conventional moldboard plowing (MP). However, the depth distribution of soil C and N varied with tillage. In the surface 0–10cm, C and N contents were higher under NT than under MP, whereas at deeper levels (20–40cm) the reverse trend was observed. It is concluded than under eastern Canadian conditions, where crop production and residue inputs are not affected by tillage, reduced tillage systems would not result in the storage of more soil organic matter in the entire soil profile at least in a 5–10 year period. Placement of the residues would be a major factor influencing the C and N distribution at specific depths.