6,884 research outputs found
Identification region of the potential outcome distributions under instrument independence
This paper examines identification power of the instrument exogeneity assumption in the treatment effect model. We derive the identification region: The set of potential outcome distributions that are compatible with data and the model restriction. The model restrictions whose identifying power is investigated are (i)instrument independence of each of the potential outcome (marginal independence), (ii) instrument joint independence of the potential outcomes and the selection heterogeneity, and (iii) instrument monotonicity in addition to (ii) (the LATE restriction of Imbens and Angrist (1994)), where these restrictions become stronger in the order of listing. By comparing the size of the identification region under each restriction, we show that the joint independence restriction can provide further identifying information for the potential outcome distributions than marginal independence, but the LATE restriction never does since it solely constrains the distribution of data. We also derive the tightest possible bounds for the average treatment effects under each restriction. Our analysis covers both the discrete and continuous outcome case, and extends the treatment effect bounds of Balke and Pearl(1997) that are available only for the binary outcome case to a wider range of settings including the continuous outcome case
A Test for Instrument Validity
This paper develops a specification test for instrument validity in the heterogeneous treatment effect model with a binary treatment and a discrete instrument. The strongest testable implication for instrument validity is given by the condition for nonnegativity of point‐identifiable compliers' outcome densities. Our specification test infers this testable implication using a variance‐weighted Kolmogorov–Smirnov test statistic. The test can be applied to both discrete and continuous outcome cases, and an extension of the test to settings with conditioning covariates is provided
The origin of the Bernoulli numbers: mathematics in Basel and Edo in the early eighteenth century
The Bernoulli numbers were named after the Swiss mathematician Jacob Bernoulli (1654–1705; Figure 1), whose posthumous book Ars Conjectandi (1713) demonstrated the calculation of sums of integer powers. The same sequences of numbers were also published in the Japanese capital, Edo (today Tokyo), also as a posthumous publication, by Takakazu Seki (?–1708), appearing one year prior to Bernoulli’s book, and so these numbers might just as well have become known as Seki numbers. This paper introduces the parallel development of mathematics in Basel, Switzerland, and Edo, Japan, and highlights the global conditions that allowed for multiple origins of mathematical discoveries
The identification region of the potential outcome distributions under instrument independence
This paper examines the identifying power of instrument exogeneity in the treatment effect model. We derive the identification region of the potential outcome distributions, which are the collection of distributions that are compatible with data and with the restrictions of the model. We consider identification when (i) the instrument is independent of each of the potential outcomes (marginal independence), (ii) the instrument is independent of the potential outcomes and selection heterogeneity jointly (joint independence), and (iii.) the instrument satisfies joint independence and monotonicity (the LATE restriction). By comparing the size of the identification region under each restriction, we show that joint independence provides more identifying information for the potential outcome distributions than marginal independence, but that the LATE restriction provides no identification gain beyond joint independence. We also, under each restriction, derive sharp bounds for the Average Treatment Effect and sharp testable implication to falsify the restriction. Our analysis covers discrete or continuous outcomes, and extends the Average Treatment Effect bounds of Balke and Pearl (1997) developed for the dichotomous outcome case to a more general setting
S wave superconductivity in newly discovered superconductor BaTiSbO revealed by Sb-NMR/Nuclear Quadrupole Resonance measurements
We report the Sb-NMR/nuclear quadrupole resonance (NQR)
measurements on the newly-discovered superconductor BaTiSbO with a
two-dimensional TiO square-net layer formed with Ti (3). NQR
measurements revealed that the in-plane four-fold symmetry is broken at the Sb
site below 40 K, without an internal field appearing at the Sb
site. These exclude a spin-density wave (SDW)/ charge density wave (CDW)
ordering with incommensurate correlations, but can be understood with the
commensurate CDW ordering at . The spin-lattice relaxation rate
, measured at the four-fold symmetry breaking site, decreases below
superconducting (SC) transition temperature , indicative of the
microscopic coexistence of superconductivity and the CDW/SDW phase below
. Furthermore, of Sb-NQR shows a coherence peak just
below and decreases exponentially at low temperatures. These
results are in sharp contrast with those in cuprate and iron-based
superconductors, and strongly suggest that its SC symmetry is classified to an
ordinary s-wave state.Comment: 5 pages, 6 figure
Universality and Critical Behavior at the Critical-End-Point on Itinerant-Metamagnet UCoAl
We performed nuclear-magnetic-resonance (NMR) measurements on
itinerant-electron metamagnet UCoAl in order to investigate the critical
behavior of the magnetism near a metamagnetic (MM) critical endpoint (CEP). We
derived c-axis magnetization and its fluctuation from the
measurements of Knight shift and nuclear spin-lattice relaxation rate
as a function of the c-axis external field () and temperature (). We
developed contour plots of and on the - phase diagram,
and observed the strong divergence of at the CEP. The critical exponents
of and near the CEP are estimated, and found to be close to the
universal properties of a three-dimensional (3-D) Ising model. We indicate that
the critical phenomena at the itinerant-electron MM CEP in UCoAl have a common
feature as a gas-liquid transition.Comment: 8 Pages, 14 figure
Magnetic and superconducting properties on S-type single-crystal CeCuSi probed by Cu nuclear magnetic resonance and nuclear quadrupole resonance
We have performed Cu nuclear magnetic resonance/nuclear quadrupole
resonance measurements to investigate the magnetic and superconducting (SC)
properties on a "superconductivity dominant" (-type) single crystal of
CeCuSi. Although the development of antiferromagnetic (AFM)
fluctuations down to 1~K indicated that the AFM criticality was close, Korringa
behavior was observed below 0.8~K, and no magnetic anomaly was observed above
0.6 K. These behaviors were expected in -type
CeCuSi. The temperature dependence of the nuclear spin-lattice
relaxation rate at zero field was almost identical to that in the
previous polycrystalline samples down to 130~mK, but the temperature dependence
deviated downward below 120~mK. In fact, in the SC state could be
fitted with the two-gap -wave rather than the two-gap -wave
model down to 90~mK. Under magnetic fields, the spin susceptibility in both
directions clearly decreased below , indicative of the formation of
spin singlet pairing. The residual part of the spin susceptibility was
understood by the field-induced residual density of states evaluated from
, which was ascribed to the effect of the vortex cores. No magnetic
anomaly was observed above the upper critical field , but the
development of AFM fluctuations was observed, indicating that superconductivity
was realized in strong AFM fluctuations.Comment: 10 pages, 8 figure
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