160 research outputs found

    Geographical Features vs. Institutional Factors : New Perspectives on the Growth of Africa and Middle-East

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    Economic Research Forum Working paper n°490International audienceThis paper examines Africa's and Middle East's growth performance for the period 1990- 2005. It employs a Bayesian Model Averaging method that constructs estimates as a weighted average of Spatial Autoregressive estimates for every possible combination of included variables. One of the results of the paper is that the inclusion of spatial dependencies has a direct impact on the determinants of growth in Africa and Middle-East. Indeed, the methodology used in the paper offers an interesting response to the institution/geography debate on the explanation of growth and development. In particular, our methodology allows a selection of the institutional variables that count to explain low development since the geographical variables are partially integrated in the spatial dependence effect

    Characterization of random stress fields obtained from polycrystalline aggregate calculations using multi-scale stochastic finite elements

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    The spatial variability of stress fields resulting from polycrystalline aggregate calculations involving random grain geometry and crystal orientations is investigated. A periodogram-based method is proposed to identify the properties of homogeneous Gaussian random fields (power spectral density and related covariance structure). Based on a set of finite element polycrystalline aggregate calculations the properties of the maximal principal stress field are identified. Two cases are considered, using either a fixed or random grain geometry. The stability of the method w.r.t the number of samples and the load level (up to 3.5 % macroscopic deformation) is investigated

    Observational biases in Lagrangian reconstructions of cosmic velocity fields

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    Lagrangian reconstruction of large-scale peculiar velocity fields can be strongly affected by observational biases. We develop a thorough analysis of these systematic effects by relying on specially selected mock catalogues. For the purpose of this paper, we use the MAK reconstruction method, although any other Lagrangian reconstruction method should be sensitive to the same problems. We extensively study the uncertainty in the mass-to-light assignment due to luminosity incompleteness, and the poorly-determined relation between mass and luminosity. The impact of redshift distortion corrections is analyzed in the context of MAK and we check the importance of edge and finite-volume effects on the reconstructed velocities. Using three mock catalogues with different average densities, we also study the effect of cosmic variance. In particular, one of them presents the same global features as found in observational catalogues that extend to 80 Mpc/h scales. We give recipes, checked using the aforementioned mock catalogues, to handle these particular observational effects, after having introduced them into the mock catalogues so as to quantitatively mimic the most densely sampled currently available galaxy catalogue of the nearby universe. Once biases have been taken care of, the typical resulting error in reconstructed velocities is typically about a quarter of the overall velocity dispersion, and without significant bias. We finally model our reconstruction errors to propose an improved Bayesian approach to measure Omega_m in an unbiased way by comparing the reconstructed velocities to the measured ones in distance space, even though they may be plagued by large errors. We show that, in the context of observational data, a nearly unbiased estimator of Omega_m may be built using MAK reconstruction.Comment: 29 pages, 21 figures, 6 tables, Accepted by MNRAS on 2007 October 2. Received 2007 September 30; in original form 2007 July 2

    Basal mechanics of ice streams: Insights from the stick-slip motion of Whillans Ice Stream, West Antarctica

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    The downstream portion of Whillans Ice Stream, West Antarctica, moves primarily by stick-slip motion. The observation of stick-slip motion suggests that the bed is governed by velocity-weakening physics and that the basal physics is more unstable than suggested by laboratory studies. The stick-slip cycle of Whillans Ice Plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude. To understand this variability, we modeled the forces acting on the ice stream during the stick phase of the stick-slip cycle. The ocean tides introduce changes in the rate at which stress is applied to the ice plain. Increased loading rates promote earlier failure and vice versa. Results show that the bed of Whillans Ice Stream strengthens over time (healing) during the quiescent intervals in the stick-slip cycle, with the bed weakening during slip events. The time-dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0.35 kPa during the course of a single day

    APPLICATIONS OF STATISTICAL METHODS STUDYING THE IMPACT OF MOBILIZATION REGIMES ON THE TOTAL COLLECTION YIELD OF HEMATOLOGIC STEM CELL

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    Hematologic malignancies are cancers that develop in the blood, bone marrow, and lymph nodes. Hematopoietic stem cell transplantation (HSCT) is a critical therapeutic approach that contributes to offer a potential cure for hematologic cancers and other hematologic disorders by replacing abnormal bone marrow with healthy bone marrow components to help bone marrow function recovery. Peripheral blood is the primary resource for collecting hematopoietic stem cells (HSCs). The collection yield of HSCs is critical for successful transplantation. Few articles have discussed this topic that the collection of stem cells not only from healthy donors but also from donors with a hematologic or non-hematologic malignancy who likewise be in danger of mobilization failure. Therefore, our goal of this paper is to study the collection efficiency of obtaining HSCs. Correlations were measured by computing Spearman’s correlation coefficients. Trends were measured using the Jonckheere-Terpstra test. Differences in groups were measured using the Mann-Whitney U test. An all subsets selection model was built to compare to the model built with a purposeful variable selection method. We employed sensitivity analysis to compare the models and find the factors that influence the efficient collection. Our results showed that many factors contribute to an efficient collection (r2 = 0.6). However, all but one of these factors correlated very poorly with collection efficiency. It was the predicted total number of CD34+ progenitor cells that correlated most strongly with predicting collection efficiency. Our results showed that the patient\u27s CD34+ cells could keep up at a sufficient level even after large volume apheresis. Also, adding or deduct drug (plerixafor) usage could cause the patient’s CD34+ level to increase or decrease. Overall, these discoveries will help to determine the mobilizing drug usages when harvesting HSCs

    The Sources of Macroeconomic Fluctuations in Subsaharan African Economies: An application to Côte d'Ivoire

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    This paper quantifies the empirical importance of various types of relevant shocks in explaining macroeconomic uctuations in a typical Sub{saharan African economy (C^ote d'Ivoire) in the context of a Dynamic Stochastic General Equilibrium (DSGE) model and Bayesian techniques. Our analysis first documents that transitory but persistent productivity shocks are the dominant sources of macroeconomic volatility as they explain more than half of aggregate uctuations. Second, world interest rate shocks are found to be non{negligible especially in driving uctuations in consumption growth. Third, while fiscal policy is found to be procyclical, fiscal shocks play a minor role in this economy. In addition, negative productivity shocks coupled with positive world interest rate shocks are at the origins of the poor macroeconomic performances of the economy in the 80s. These findings are in line with the business cycle literature on African economies and also robust to various perturbations of the benchmark set-up

    The Sources of Macroeconomic Fluctuations in Subsaharan African Economies: An application to Côte d'Ivoire

    Get PDF
    This paper quantifies the empirical importance of various types of relevant shocks in explaining macroeconomic uctuations in a typical Sub{saharan African economy (C^ote d'Ivoire) in the context of a Dynamic Stochastic General Equilibrium (DSGE) model and Bayesian techniques. Our analysis first documents that transitory but persistent productivity shocks are the dominant sources of macroeconomic volatility as they explain more than half of aggregate uctuations. Second, world interest rate shocks are found to be non{negligible especially in driving uctuations in consumption growth. Third, while fiscal policy is found to be procyclical, fiscal shocks play a minor role in this economy. In addition, negative productivity shocks coupled with positive world interest rate shocks are at the origins of the poor macroeconomic performances of the economy in the 80s. These findings are in line with the business cycle literature on African economies and also robust to various perturbations of the benchmark set-up

    Generalization of the noise model for time-distance helioseismology

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    In time-distance helioseismology, information about the solar interior is encoded in measurements of travel times between pairs of points on the solar surface. Travel times are deduced from the cross-covariance of the random wave field. Here we consider travel times and also products of travel times as observables. They contain information about e.g. the statistical properties of convection in the Sun. The basic assumption of the model is that noise is the result of the stochastic excitation of solar waves, a random process which is stationary and Gaussian. We generalize the existing noise model (Gizon and Birch 2004) by dropping the assumption of horizontal spatial homogeneity. Using a recurrence relation, we calculate the noise covariance matrices for the moments of order 4, 6, and 8 of the observed wave field, for the moments of order 2, 3 and 4 of the cross-covariance, and for the moments of order 2, 3 and 4 of the travel times. All noise covariance matrices depend only on the expectation value of the cross-covariance of the observed wave field. For products of travel times, the noise covariance matrix consists of three terms proportional to 1/T1/T, 1/T21/T^2, and 1/T31/T^3, where TT is the duration of the observations. For typical observation times of a few hours, the term proportional to 1/T21/T^2 dominates and Cov[τ1τ2,τ3τ4]Cov[τ1,τ3]Cov[τ2,τ4]+Cov[τ1,τ4]Cov[τ2,τ3]Cov[\tau_1 \tau_2, \tau_3 \tau_4] \approx Cov[\tau_1, \tau_3] Cov[\tau_2, \tau_4] + Cov[\tau_1, \tau_4] Cov[\tau_2, \tau_3], where the τi\tau_i are arbitrary travel times. This result is confirmed for p1p_1 travel times by Monte Carlo simulations and comparisons with SDO/HMI observations. General and accurate formulae have been derived to model the noise covariance matrix of helioseismic travel times and products of travel times. These results could easily be generalized to other methods of local helioseismology, such as helioseismic holography and ring diagram analysis

    Logical indetermination coupling: a method to minimize drawing matches and its applications

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    While justifying that independence is a canonic coupling, the authors show the existence of a second equilibrium to reduce the information conveyed from the margins to the joined distribution: the so-called indetermination. They use this null information property to apply indetermination to graph clustering. Furthermore, they break down a drawing under indetermination to emphasis it is the best construction to reduce couple matchings, meaning, the expected number of equal couples drawn in a row. Using this property, they notice that indetermination appears in two problems (Guessing and Task Partitioning) where couple matchings reduction is a key objective.Comment: arXiv admin note: text overlap with arXiv:2007.0882
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