Instrumental-Variables Simultaneous Equations Model of Physical Activity and Body Mass Index: The Coronary Artery Risk Development in Young Adults (CARDIA) Study

We used full-system-estimation instrumental-variables simultaneous equations modeling (IV-SEM) to examine physical activity relative to body mass index (BMI; weight (kg)/height (m)(2)) using 25 years of data (1985/1986 to 2010/2011) from the Coronary Artery Risk Development in Young Adults (CARDIA) Study (n = 5,115; ages 18-30 years at enrollment). Neighborhood environment and sociodemographic instruments were used to characterize physical activity, fast-food consumption, smoking, alcohol consumption, marriage, and childbearing (women) and to predict BMI using semiparametric full-information maximum likelihood estimation to control for unobserved time-invariant and time-varying residual confounding and differential measurement error through model-derived discrete random effects. Comparing robust-variance ordinary least squares, random-effects regression, fixed-effects regression, single-equation-estimation IV-SEM, and full-system-estimation IV-SEM, estimates from random- and fixed-effects models and the full-system-estimation IV-SEM were unexpectedly similar, despite the lack of control for residual confounding with the random-effects estimator. Ordinary least squares tended to overstate the significance of health behaviors in BMI, while results from single-equation-estimation IV-SEM were notably different, revealing the impact of weak instruments in standard instrumental-variable methods. Our robust findings for fixed effects (which does not require instruments but has a high cost in lost degrees of freedom) and full-system-estimation IV-SEM (vs. standard IV-SEM) demonstrate potential for a full-system-estimation IV-SEM method even with weak instruments

Changes in marine sedimentation patterns in the northeastern South China Sea in the past 35,000 years

International audienceIn the continental margin of the northeastern South China Sea, the sea level fluctuations since the Last Glacial Maximum have profoundly impacted the sedimentary environment. Our sub-bottom profiler data show a sedimentation process change from deposition to erosion during the Last Glacial Maximum. After the widespread erosion, the sedimentation process returned depositional throughout the Holocene, probably due to the rise of the sea level. This sedimentary process results in a widespread sedimentary unconformity in the continental slope, providing a benchmark for the end of the Last Glacial Maximum. Analyzing the sediment cores, we affirm that the change in current intensity is the primary factor controlling the sedimentary environments. The current intensities strengthened during the eustatic lowstand and weakened during the highstand periods, leading to alternating erosional and depositional processes. The widespread distribution of the erosive surface represents a regional-scale change in the sedimentary environment instead of a local event

Changes in marine sedimentation patterns in the northeastern South China Sea in the past 35,000 years

International audienceIn the continental margin of the northeastern South China Sea, the sea level fluctuations since the Last Glacial Maximum have profoundly impacted the sedimentary environment. Our sub-bottom profiler data show a sedimentation process change from deposition to erosion during the Last Glacial Maximum. After the widespread erosion, the sedimentation process returned depositional throughout the Holocene, probably due to the rise of the sea level. This sedimentary process results in a widespread sedimentary unconformity in the continental slope, providing a benchmark for the end of the Last Glacial Maximum. Analyzing the sediment cores, we affirm that the change in current intensity is the primary factor controlling the sedimentary environments. The current intensities strengthened during the eustatic lowstand and weakened during the highstand periods, leading to alternating erosional and depositional processes. The widespread distribution of the erosive surface represents a regional-scale change in the sedimentary environment instead of a local event