49 research outputs found
The impact of chorionicity on pregnancy outcome and neurodevelopment at 2 years old among twins born preterm: the EPIPAGE-2 cohort study
OBJECTIVE
To compare the shortâ and midâterm outcomes of preterm twins by chorionicity of pregnancy.
DESIGN
Prospective nationwide populationâbased EPIPAGEâ2 cohort study.
SETTING
546 maternity units in France, between March and December 2011.
POPULATION
A total of 1700 twin neonates born between 24 and 34 weeks of gestation.
METHODS
The association of chorionicity with outcomes was analysed using multivariate regression models.
MAIN OUTCOME MEASURES
First, survival at 2âyear corrected age with or without neurosensory impairment, and second, perinatal, shortâ, and midâterm outcomes (survival at discharge, survival at discharge without severe morbidity) were described and compared by chorionicity.
RESULTS
In the EPIPAGE 2 cohort, 1700 preterm births were included (850 twin pregnancies). In all, 1220 (71.8%) were from dichorionic (DC) pregnancies and 480 from monochorionic (MC) pregnancies. MC pregnancies had three times more medical terminations than DC pregnancies (1.67 versus 0.51%, P < 0.001), whereas there were three times more stillbirths in MC than in DC pregnancies (10.09 versus 3.78%, P < 0.001). Both twins were alive at birth in 86.6% of DC pregnancies compared with 80.0% among MC pregnancies (P = 0.008). No significant difference according to chorionicity was found regarding neonatal deaths and morbidities. Likewise, for children born earlier than 32 weeks, the 2âyear followâup neurodevelopmental results were not significantly different between DC and MC twins.
CONCLUSIONS
This study confirms that MC pregnancies have a higher risk of adverse outcomes. However, the outcomes among preterm twins admitted to neonatal intensive care units are similar irrespective of chorionicity
On the crystal lattice parameters of graphite-like phases of the B-C system
The structure of graphite-like BCx phases (x = 1, 1.5, 3, 4, 32) has been
studied using conventional X-ray diffraction. The results have been obtained,
which unambiguously point to turbostratic (one- dimensionally disordered)
structure of all phases under study. The crystal lattice parameters, sizes of
coherent scattering domains, and microstrain values have been defined, which
have allowed us to find a correlation between the structure and stoichiometry
of the phases synthesized at the same temperature
Evaluation of a weighting approach for performing sensitivity analysis after multiple imputation
Abstract
Background
Multiple imputation (MI) is a well-recognised statistical technique for handling missing data. As usually implemented in standard statistical software, MI assumes that data are âMissing at randomâ (MAR); an assumption that in many settings is implausible. It is not possible to distinguish whether data are MAR or âMissing not at randomâ (MNAR) using the observed data, so it is desirable to discover the impact of departures from the MAR assumption on the MI results by conducting sensitivity analyses. A weighting approach based on a selection model has been proposed for performing MNAR analyses to assess the robustness of results obtained under standard MI to departures from MAR.
Methods
In this article, we use simulation to evaluate the weighting approach as a method for exploring possible departures from MAR, with missingness in a single variable, where the parameters of interest are the marginal mean (and probability) of a partially observed outcome variable and a measure of association between the outcome and a fully observed exposure. The simulation studies compare the weighting-based MNAR estimates for various numbers of imputations in small and large samples, for moderate to large magnitudes of departure from MAR, where the degree of departure from MAR was assumed known. Further, we evaluated a proposed graphical method, which uses the dataset with missing data, for obtaining a plausible range of values for the parameter that quantifies the magnitude of departure from MAR.
Results
Our simulation studies confirm that the weighting approach outperformed the MAR approach, but it still suffered from bias. In particular, our findings demonstrate that the weighting approach provides biased parameter estimates, even when a large number of imputations is performed. In the examples presented, the graphical approach for selecting a range of values for the possible departures from MAR did not capture the true parameter value of departure used in generating the data.
Conclusions
Overall, the weighting approach is not recommended for sensitivity analyses following MI, and further research is required to develop more appropriate methods to perform such sensitivity analyses
Large scale flows under location uncertainty: a consistent stochastic framework
International audienceUsing a classical example, the Lorenz-63 model, an original stochastic framework is applied to represent large-scale geophysical flow dynamics. Rigorously derived from a reformulated material derivative, the proposed framework encompasses several meaningful mechanisms to model geophysical flows. The slightly compressible setup , as treated in the Boussinesq approximation, brings up a stochastic transport equation for the density and other related thermo-dynamical variables. Coupled to the momentum equation through a forcing term, a resulting stochastic Lorenz-63 model is consistently derived. Based on such a reformulated model, the pertinence of this large-scale stochastic approach is demonstrated over classical eddy-viscosity based large-scale representations
Validity domains and parametrizations for white-noise and multiscale models in turbulence and wave-turbulence interactions
International audienc
Validity domains and parametrizations for white-noise and multiscale models in turbulence and wave-turbulence interactions
International audienc
Ăvaluation du transport sĂ©dimentaire sur une plage macrotidale par traçage fluorescent - Dunkerque Est - France
Une expérience d'estimation des transports sédimentaires sur une plage macrotidale du littoral est-dunkerquois (Nord de la France) a été réalisée du 19 au 27 avril 1993 au moyen de traceur fluorescent détecté à l'aide d'un compteur automatique. Les conditions hydrodynamiques se rapportent à une marée de petite vive-eau avec des vents faibles à modérés. En période de vents faibles, l'action du flot est prépondérante et conduit à un transport de sable parallÚle à la cÎte, dirigé vers l'Es