101,415 research outputs found
The correlation structure of spatial autoregressions
This paper investigates how the correlations implied by a first-order simultaneous
autoregressive (SAR(1)) process are affected by the weights matrix and the autocorrelation
parameter. A graph theoretic representation of the covariances in terms of
walks connecting the spatial units helps to clarify a number of correlation properties
of the processes. In particular, we study some implications of row-standardizing
the weights matrix, the dependence of the correlations on graph distance, and the
behavior of the correlations at the extremes of the parameter space. Throughout
the analysis differences between directed and undirected networks are emphasized.
The graph theoretic representation also clarifies why it is difficult to relate properties
ofW to correlation properties of SAR(1) models defined on irregular lattices
Representations of sources and data: working with exceptions to hierarchy in historical documents
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Risk as reward: Reinforcement sensitivity theory and psychopathic personality perspectives on everyday risk-taking
This study updates and synthesises research on the extent to which impulsive and antisocial disposition predicts everyday pro- and antisocial risk-taking behaviour. We use the Reinforcement Sensitivity Theory (RST) of personality to measure approach, avoidance, and inhibition dispositions, as well as measures of Callous-Unemotional and psychopathic personalities. In an international sample of 454 respondents, results showed that RST, psychopathic personality, and callous-unemotional measures accounted for different aspects of risk-taking behaviour. Specifically, traits associated with ‘fearlessness’ related more to ‘prosocial’ (recreational and social) risk-taking, whilst traits associated with ‘impulsivity’ related more to ‘antisocial’ (ethical and health) risk-taking. Further, we demonstrate that psychopathic personality may be demonstrated by combining the RST and callous-unemotional traits (high impulsivity, callousness, and low fear). Overall this study showed how impulsive, fearless and antisocial traits can be used in combination to identify pro- and anti-social risk-taking behaviours; suggestions for future research are indicated
Radiocarbon Date List XI: Radiocarbon Dates from Marine Sediment Cores of the Iceland, Greenland, and Northeast Canadian Arctic Shelves and Nares Strait
Radiocarbon Date List XI contains an annotated listing of 178 AMS radiocarbon dates on samples from marine (169 samples) and lake (9 samples) sediment cores. Marine sediment cores, from which the samples for dating were taken, were collected on the Greenland Shelf, Baffin Bay, and the Eastern Canadian Arctic shelf. About 80% of the marine samples for dating were collected on the SW to N Icelandic shelf. The lake sediment cores were collected in northwestern Iceland. For dating of the marine samples, we submitted molluscs (117 samples), benthic and planktic foraminifera (45 samples), plant macrofauna (3 samples), and one serpulid worm. For dating of the lake cores, we submitted wood (8 samples) and one peat sample. The Conventional Radiocarbon Ages range from 294±9114C yr BP to 34,600±640 14C yr BP. The dates have been used to address a variety of research questions. The dates constrain the timing of high northern latitude late Quaternary environmental fluctuations, which include glacier extent, sea level history, isostatic rebound, sediment input, and ocean circulation. The dates also allowed assessment of the accuracy of commonly used reservoir correction. The samples were submitted by INSTAAR and affiliated researchers
Variability-selected low-luminosity active galactic nuclei candidates in the 7 Ms Chandra Deep Field-South
In deep X-ray surveys, active galactic nuclei (AGNs) with a broad range of
luminosities have been identified. However, cosmologically distant
low-luminosity AGN (LLAGN, erg s)
identification still poses a challenge due to significant contamination from
host galaxies. Based on the 7 Ms Chandra Deep Field-South (CDF-S) survey, the
longest timescale ( years) deep X-ray survey to date, we utilize an
X-ray variability selection technique to search for LLAGNs that remain
unidentified among the CDF-S X-ray sources. We find 13 variable sources from
110 unclassified CDF-S X-ray sources. Except for one source which could be an
ultraluminous X-ray source, the variability of the remaining 12 sources is most
likely due to accreting supermassive black holes. These 12 AGN candidates have
low intrinsic X-ray luminosities, with a median value of erg
s. They are generally not heavily obscured, with an average effective
power-law photon index of 1.8. The fraction of variable AGNs in the CDF-S is
independent of X-ray luminosity and is only restricted by the total number of
observed net counts, confirming previous findings that X-ray variability is a
near-ubiquitous property of AGNs over a wide range of luminosities. There is an
anti-correlation between X-ray luminosity and variability amplitude for
high-luminosity AGNs, but as the luminosity drops to erg
s, the variability amplitude no longer appears dependent on the
luminosity. The entire observed luminosity-variability trend can be roughly
reproduced by an empirical AGN variability model based on a broken power-law
power spectral density function.Comment: 18 pages, 11 figures, accepted for publication in Ap
Non-pecuniary returns to higher education: the effect on smoking intensity in the UK
This paper investigates whether higher education (HE) produces non-pecuniary returns via a reduction in the
intensity of consumption of health-damaging substances. In particular, it focuses on current smoking intensity of the British individuals sampled in the 29-year follow-up survey of the 1970 British Cohort Study. We estimate endogenous dummy ordinal response models for cigarette consumption and show that HE is endogenous with respect to smoking intensity and that even when endogeneity is accounted for, HE is found to have a strong negative effect on smoking intensity. Moreover, pecuniary channels, such as occupation and income, mediate only a minor part of the effect of HE. Our results are robust to modelling individual self-selection into current smoking
participation (at age 29) and to estimating a dynamic model in which past smoking levels affect current smoking levels
Spectral absorption of biomass burning aerosol determined from retrieved single scattering albedo during ARCTAS
Actinic flux, as well as aerosol chemical and optical properties, were measured aboard the NASA DC-8 aircraft during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) mission in Spring and Summer 2008. These measurements were used in a radiative transfer code to retrieve spectral (350-550 nm) aerosol single scattering albedo (SSA) for biomass burning plumes encountered on 17 April and 29 June. Retrieved SSA values were subsequently used to calculate the absorption Angstrom exponent (AAE) over the 350-500 nm range. Both plumes exhibited enhanced spectral absorption with AAE values that exceeded 1 (6.78 ± 0.38 for 17 April and 3.34 ± 0.11 for 29 June). This enhanced absorption was primarily due to organic aerosol (OA) which contributed significantly to total absorption at all wavelengths for both 17 April (57.7%) and 29 June (56.2%). OA contributions to absorption were greater at UV wavelengths than at visible wavelengths for both cases. Differences in AAE values between the two cases were attributed to differences in plume age and thus to differences in the ratio of OA and black carbon (BC) concentrations. However, notable differences between AAE values calculated for the OA (AAEOA) for 17 April (11.15 ± 0.59) and 29 June (4.94 ± 0.19) suggested differences in the plume AAE values might also be due to differences in organic aerosol composition. The 17 April OA was much more oxidized than the 29 June OA as denoted by a higher oxidation state value for 17 April (+0.16 vs. -0.32). Differences in the AAEOA, as well as the overall AAE, were thus also possibly due to oxidation of biomass burning primary organic aerosol in the 17 April plume that resulted in the formation of OA with a greater spectral-dependence of absorption. © Author(s) 2012. CC Attribution 3.0 License
Instrumental Variables Estimation with Some Invalid Instruments and its Application to Mendelian Randomization
Instrumental variables have been widely used for estimating the causal effect
between exposure and outcome. Conventional estimation methods require complete
knowledge about all the instruments' validity; a valid instrument must not have
a direct effect on the outcome and not be related to unmeasured confounders.
Often, this is impractical as highlighted by Mendelian randomization studies
where genetic markers are used as instruments and complete knowledge about
instruments' validity is equivalent to complete knowledge about the involved
genes' functions.
In this paper, we propose a method for estimation of causal effects when this
complete knowledge is absent. It is shown that causal effects are identified
and can be estimated as long as less than % of instruments are invalid,
without knowing which of the instruments are invalid. We also introduce
conditions for identification when the 50% threshold is violated. A fast
penalized estimation method, called sisVIVE, is introduced for
estimating the causal effect without knowing which instruments are valid, with
theoretical guarantees on its performance. The proposed method is demonstrated
on simulated data and a real Mendelian randomization study concerning the
effect of body mass index on health-related quality of life index. An R package
\emph{sisVIVE} is available online.Comment: 99 pages, 29 figures, 14 table
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