Comparison of the Inverse Probability of Treatment Weighted (IPTW) Estimator With a Naïve Estimator in the Analysis of Longitudinal Data With Time-Dependent Confounding: A Simulation Study

A simulation study was conducted to compare estimates from a naïve estimator, using standard conditional regression, and an IPTW (Inverse Probability of Treatment Weighted) estimator, to true causal parameters for a given MSM (Marginal Structural Model). The study was extracted from a larger epidemiological study (Longitudinal Study of Effects of Physical Activity and Body Composition on Functional Limitation in the Elderly, by Tager et. al [accepted, Epidemiology, September 2003]), which examined the causal effects of physical activity and body composition on functional limitation. The simulation emulated the larger study in terms of the exposure and outcome variables of interest-- physical activity (LTPA), body composition (LNFAT), and physical limitation (PF), but used one time-dependent confounder (HEALTH) to illustrate the effects of estimating causal effects in the presence of time-dependent confounding. In addition to being a time-dependent confounder (i.e. predictor of exposure and outcome over time), HEALTH was also affected by past treatment. Under these conditions, naïve estimates are known to give biased estimates of the causal effects of interest (Robins, 2000). The true causal parameters for LNFAT (-0.61) and LTPA (-0.70) were obtained by assessing the log-odds of functional limitation for a 1-unit increase in LNFAT and participation in vigorous exercise in an ideal experiment in which the counterfactual outcomes were known for every possible combination of LNFAT and LTPA for each subject. Under conditions of moderate confounding, the IPTW estimates for LNFAT and LTPA were -0.62 and -0.94, respectively, versus the naïve estimates of -0.78 and -0.80. For increased levels of confounding of the LNFAT and LTPA variables, the IPTW estimates were -0.60 and -1.28, respectively, and the naïve estimates were -0.85 and -0.87. The bias of the IPTW estimates, particularly under increased levels of confounding, was explored and linked to violation of particular assumptions regarding the IPTW estimation of causal parameters for the MSM

Lesions of the paraventricular nucleus of the thalamus differentially affect sign‐ and goal‐tracking conditioned responses

Recently, evidence has emerged suggesting a role for the paraventricular nucleus of the thalamus (PVT) in the processing of reward‐associated cues. However, the specific role of the PVT in these processes has yet to be elucidated. Here we use an animal model that captures individual variation in response to discrete reward‐associated cues to further assess the role of the PVT in stimulus–reward learning. When rats are exposed to a Pavlovian conditioning paradigm, wherein a discrete cue predicts food reward, two distinct conditioned responses emerge. Some rats, termed sign‐trackers, approach and manipulate the cue, whereas others, termed goal‐trackers, approach the location of reward delivery upon cue presentation. For both sign‐ and goal‐trackers the cue is a predictor, but only for sign‐trackers is it also an incentive stimulus. We investigated the role of the PVT in the acquisition and expression of these conditioned responses using an excitotoxic lesion. Results indicate that PVT lesions prior to acquisition amplify the differences between phenotypes – increasing sign‐tracking and attenuating goal‐tracking behavior. Lesions of the PVT after rats had acquired their respective conditioned responses also attenuated the expression of the goal‐tracking response, and increased the sign‐tracking response, but did so selectively in goal‐trackers. These results suggest that the PVT acts to suppress the attribution of incentive salience to reward cues, as disruption of the functional activity within this structure enhances the tendency to sign‐track.Here we utilized animal models that capture individual differences in the propensity to attribute incentive salience to reward cues (i.e. sign‐trackers vs. goal‐trackers) to further elucidate the role of the PVT in cue‐motivated behaviors. We report that lesions of this structure increase the tendency for individuals to attribute incentive motivational value to reward cues. These findings suggest that the PVT is a critical part of the circuitry underlying maladaptive behavior, such as addiction.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115973/1/ejn13031-sup-0001-TableS1-FigureS1-S5.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/115973/2/ejn13031.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/115973/3/ejn13031_am.pd

The 14C(n,g) cross section between 10 keV and 1 MeV

The neutron capture cross section of 14C is of relevance for several nucleosynthesis scenarios such as inhomogeneous Big Bang models, neutron induced CNO cycles, and neutrino driven wind models for the r process. The 14C(n,g) reaction is also important for the validation of the Coulomb dissociation method, where the (n,g) cross section can be indirectly obtained via the time-reversed process. So far, the example of 14C is the only case with neutrons where both, direct measurement and indirect Coulomb dissociation, have been applied. Unfortunately, the interpretation is obscured by discrepancies between several experiments and theory. Therefore, we report on new direct measurements of the 14C(n,g) reaction with neutron energies ranging from 20 to 800 keV

Network metrics can guide nearly-optimal management of invasive species at large scales

Invasive species harm biodiversity and ecosystem services, with global economic costs of invasions exceeding $40 billion annually. Widespread invasions are a particular challenge because they involve large spatial scales with many interacting components. In these contexts, typical optimization-based approaches to management may fail due to computational or data constraints. Here we evaluate an alternative solution that leverages network science, representing the invasion as occurring across a network of connected sites and using network metrics to prioritize sites for intervention. Such heuristic network-guided methods require less data and are less computationally intensive than optimization methods, yet network-guided approaches have not been bench-marked against optimal solutions for real-world invasive species management problems. We provide the first comparison of the performance of network-guided management relative to optimal solutions for invasive species, examining the placement of watercraft inspection stations for preventing spread of invasive zebra mussels through recreational boat movement within 58 Minnesota counties in the United States. To additionally test the promise of network-based approaches in limited data contexts, we evaluate their performance when using only partial data on network structure and invaded status. Metric-based approaches can achieve a median of 100% of optimal performance with full data. Even with partial data, 80% of optimal performance is achievable. Finally, we show that performance of metric-guided management improves for counties with denser and larger networks, suggesting this approach is viable for large-scale invasions. Together, our results suggest network metrics are a promising approach to guiding management actions for large-scale invasions.Comment: 29 pages, 8 figures, 3 table

Panel Discussion: Expropriation, Threats of Expropriation and Developmental Policy

This panel discussion is a conversation about the place of expropriation in international law

Chemical principles. Third edition.

PREFACE: This edition of Chemical Principles, like its predecessors, is designed to be used in a general university chemistry course which must provide both an overview of chemistry for nonspecialists and a sound foundation for later study for science or chemistry majors. Hence there are several survey chapters introducing different areas of chemistry, including inorganic, nuclear, organic, and biochemistry, and an attempt is made throughout the book to place chemistry in its historical and cultural setting. At the same time, the quantitative aspects of chemistry are presented in a manner consistent with their importance, in a way that will make it easy to build upon them in later courses. This is the first complete revision of Chemical Principles since the first edition was published in 1969. The authors have rethought and replanned the entire book, especially the first thirteen chapters, trying to make it a better pedagogical tool without losing the special viewpoints and flavor that made the earlier editions so successful. The history and the anecdotal asides that help to make the subject palatable have been retained, but they have been better segregated from the factual material for which a student will be held responsible