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## Simulações atomísticas de eventos raros através de Transition Path Sampling; Atomistic simulation of rare events using Transition Path Sampling

Fonte: Biblioteca Digital da Unicamp
Publicador: Biblioteca Digital da Unicamp

Tipo: Dissertação de Mestrado
Formato: application/pdf

Publicado em 10/09/2007
Português

Relevância na Pesquisa

67.274917%

#Simulação atomistica#Eventos raros#Metodo Transition Path Sampling#Metodo de Monte Carlo#Atomistic simulation#Rare events#Transition Path Sampling Method#Monte Carlo method

Nesta dissertação abordamos o estudo de uma das limitações da simulação atomística denominada o evento raro, quem é responsável pela limitação temporal, exemplos de problemas que envolvem os eventos raros são, o enovelamento de proteínas, mudanças conformacionais de moléculas, reações químicas (em solução), difusão de sólidos e os processos de nucleação numa transição de fase de 1a ordem, entre outros.
Métodos convencionais como Dinâmica Molecular (MD) ou Monte Carlo (MC) são úteis para explorar a paisagem de energia potencial de sistemas muito complexos, mas em presença de eventos raros se tornam muito ineficientes, devido à falta de estatística na amostragem do evento. Estes métodos gastam muito tempo computacional amostrando as configurações irrelevantes e não as transições de interesse.
Neste sentido o método Transition Path Sampling (TPS), desenvolvido por D. Chandler e seus colaboradores, consegue explorar a paisagem de energia potencial e obter um conjunto de verdadeiras trajetórias dinâmicas que conectam os estados metaestáveis em presença de evento raros. A partir do ensemble de caminhos a constante de reação e o mecanismo de reação podem ser extraídos com muito sucesso.
Neste trabalho de mestrado implementamos com muito sucesso o método TPS e realizamos uma comparação quantitativa em relação ao método MC configuracional num problema padrão da isomerização de uma molécula diatômica imersa num líquido repulsivo tipo Weeks-Chandler-Andersen (WCA). A aplicação destes métodos mostrou como o ambiente...

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## Simulações atomísticas de eventos raros através do método super-simétrico; Atomistic simulation of rare events via the super-symmetric method

Fonte: Biblioteca Digital da Unicamp
Publicador: Biblioteca Digital da Unicamp

Tipo: Dissertação de Mestrado
Formato: application/pdf

Publicado em 03/11/2010
Português

Relevância na Pesquisa

67.205977%

#Eventos raros#Simulações atomísticas#Método super-simétrico#Dinâmica molecular#Rare events#Atomistic simultations#Super-symmetric method#Molecular dynamics

Nesta dissertação abordamos o problema da escala temporal nas simulações atomísticas, focando no problema de eventos raros. A solução deste problema so e possível com o desenvolvimento de técnicas especiais. Especificamente, estudamos o método super-simétrico para encontrar caminhos de reação. Este metodo não apresenta as limitições comuns de outros metodos para eventos raros. Aplicamos o método a três problemas padrão e encontramos que o método permite estudar as transições raras sem precisar de um conhecimento detalhado do sistema. Além disso permite observar qualitativamente os mecanismos de transição; This thesis deals with the problem of time scale in atomistic simulations, focusing on the problem of rare events. The solution to this problem is only possible with the development of special techniques. Specifically, we studied the super-symmetric method to find reaction pathways. This method does not have the usual limitations of other methods for rare events. We apply the method to three standard problems and find that the method allows to study the rare transitions without a detailed knowledge of the system. In addition, it allows us to observe qualitatively the transition mechanisms

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## Loss Due to Missing Data in Efficiency of a Locally Optimal Test for Homogeneity with Respect to Very Rare Events*

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em /10/1970
Português

Relevância na Pesquisa

46.878584%

With reference to observations of supernovae in galaxies, a locally optimal test for the hypothesis of homogeneity of the observational units with respect to occurrence of supernovae (treated as a very rare event) is obtained for the case where the data are available only for those galaxies where at least one supernova is observed. It is shown that the loss in the asymptotic efficiency of the test due to lack of reporting of galaxies with no supernovae is very heavy and in fact is infinite for the case of very rare events.

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## Of Black Swans and Tossed Coins: Is the Description-Experience Gap in Risky Choice Limited to Rare Events?

Fonte: Public Library of Science
Publicador: Public Library of Science

Tipo: Artigo de Revista Científica

Publicado em 01/06/2011
Português

Relevância na Pesquisa

46.95883%

When faced with risky decisions, people tend to be risk averse for gains and risk
seeking for losses (the reflection effect). Studies examining
this risk-sensitive decision making, however, typically ask people directly what
they would do in hypothetical choice scenarios. A recent flurry of studies has
shown that when these risky decisions include rare outcomes, people make
different choices for explicitly described probabilities than for experienced
probabilistic outcomes. Specifically, rare outcomes are overweighted when
described and underweighted when experienced. In two experiments, we examined
risk-sensitive decision making when the risky option had two equally probable
(50%) outcomes. For experience-based decisions, there was a reversal of
the reflection effect with greater risk seeking for gains than for losses, as
compared to description-based decisions. This fundamental difference in
experienced and described choices cannot be explained by the weighting of rare
events and suggests a separate subjective utility curve for experience.

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## Gauging the flexibility of the active site in soybean lipoxygenase-1 (SLO-1) through an atom-centered density matrix propagation (ADMP) treatment that facilitates the sampling of rare events

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Português

Relevância na Pesquisa

46.99793%

We present a computational methodology to sample rare events in large biological enzymes that may involve electronically polarizing, reactive processes. The approach includes simultaneous dynamical treatment of electronic and nuclear degrees of freedom, where contributions from the electronic portion are computed using hybrid density functional theory and the computational costs are reduced through a hybrid quantum mechanics/molecular mechanics (QM/MM) treatment. Thus, the paper involves a QM/MM dynamical treatment of rare events. The method is applied to probe the effect of the active site elements on the critical hydrogen transfer step in the soybean lipoxygenase-1 (SLO-1) catalyzed oxidation of linoleic acid. It is found that the dynamical fluctuations and associated flexibility of the active site are critical towards maintaining the electrostatics in the regime where the reactive process can occur smoothly. Physical constraints enforced to limit the active site flexibility are akin to mutations and, in the cases studied, have a detrimental effect on the electrostatic fluctuations, thus adversely affecting the hydrogen transfer process.

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## Adaptively biased sequential importance sampling for rare events in reaction networks with comparison to exact solutions from finite buffer dCME method

Fonte: AIP Publishing LLC
Publicador: AIP Publishing LLC

Tipo: Artigo de Revista Científica

Português

Relevância na Pesquisa

47.255728%

Critical events that occur rarely in biological processes are of great importance, but are challenging to study using Monte Carlo simulation. By introducing biases to reaction selection and reaction rates, weighted stochastic simulation algorithms based on importance sampling allow rare events to be sampled more effectively. However, existing methods do not address the important issue of barrier crossing, which often arises from multistable networks and systems with complex probability landscape. In addition, the proliferation of parameters and the associated computing cost pose significant problems. Here we introduce a general theoretical framework for obtaining optimized biases in sampling individual reactions for estimating probabilities of rare events. We further describe a practical algorithm called adaptively biased sequential importance sampling (ABSIS) method for efficient probability estimation. By adopting a look-ahead strategy and by enumerating short paths from the current state, we estimate the reaction-specific and state-specific forward and backward moving probabilities of the system, which are then used to bias reaction selections. The ABSIS algorithm can automatically detect barrier-crossing regions, and can adjust bias adaptively at different steps of the sampling process...

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## High-resolution X-ray spectroscopy of rare events: a different look at local structure and chemistry

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em 01/03/2001
Português

Relevância na Pesquisa

46.878584%

The combination of large-acceptance high-resolution X-ray optics with bright synchrotron sources permits quantitative analysis of rare events such as X-ray fluorescence from very dilute systems, weak fluorescence transitions or X-ray Raman scattering. Transition-metal Kβ fluorescence contains information about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG-1 are presented. Weaker features of the Kβ spectrum resulting from valence-level and ‘interatomic’ ligand to metal transitions contain detailed information on the ligand-atom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ray Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive low-energy (<600 eV) X-ray absorption fine structure (XAFS) spectra using hard (~10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is proportional to that of the corresponding absorption process yielding the same X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) features. XRS finds application for systems that defy XAFS analysis at low energies...

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## Logistic Regression in Rare Events Data

Fonte: Oxford University Press
Publicador: Oxford University Press

Tipo: Artigo de Revista Científica

Português

Relevância na Pesquisa

47.26447%

We study rare events data, binary dependent variables with dozens to thousands of times fewer ones (events, such as wars, vetoes, cases of political activism, or epidemiological infections) than zeros (“nonevents”). In many literatures, these variables have proven difficult to explain and predict, a problem that seems to have at least two sources. First, popular statistical procedures, such as logistic regression, can sharply underestimate the probability of rare events. We recommend corrections that outperform existing methods and change the estimates of absolute and relative risks by as much as some estimated effects reported in the literature. Second, commonly used data collection strategies are grossly inefficient for rare events data. The fear of collecting data with too few events has
led to data collections with huge numbers of observations but relatively few, and poorly measured, explanatory variables, such as in international conflict data with more than a quarter-million dyads, only a few of which are at war. As it turns out, more efficient sampling
designs exist for making valid inferences, such as sampling all available events (e.g., wars) and a tiny fraction of nonevents (peace). This enables scholars to save as much as 99% of their (nonfixed) data collection costs or to collect much more meaningful explanatory
variables.We provide methods that link these two results...

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## Essays on Asset Pricing and Econometrics

Fonte: Harvard University
Publicador: Harvard University

Tipo: Thesis or Dissertation

Português

Relevância na Pesquisa

46.99793%

This dissertation presents three essays on asset pricing and econometrics. The first chapter identifies rare events and long-run risks simultaneously from a rich data set (the Barro-Ursua macroeconomic data set) and evaluates their contributions to asset pricing in a unified framework. The proposed model of rare events and long-run risks is estimated using a Bayesian Markov-chain Monte-Carlo method, and the estimates for the disaster process are closer to the data than those in the previous studies. Major evaluation results in asset pricing include: (1) for the unleveraged annual equity premium, the predicted values are 4.8%, 4.2%, and 1.0%, respectively; (2) for the Sharpe ratio, the values are 0.72, 0.66, and 0.15, respectively.; Economics

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## Coarse-Grained Analysis of Microscopic Neuronal Simulators on Networks: Bifurcation and Rare-events computations

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/10/2010
Português

Relevância na Pesquisa

46.99793%

#Computer Science - Social and Information Networks#Nonlinear Sciences - Adaptation and Self-Organizing Systems#Physics - Biological Physics#Quantitative Biology - Neurons and Cognition

We show how the Equation-Free approach for mutliscale computations can be
exploited to extract, in a computational strict and systematic way the emergent
dynamical attributes, from detailed large-scale microscopic stochastic models,
of neurons that interact on complex networks. In particular we show how the
Equation-Free approach can be exploited to perform system-level tasks such as
bifurcation, stability analysis and estimation of mean appearance times of rare
events, bypassing the need for obtaining analytical approximations, providing
an "on-demand" model reduction. Using the detailed simulator as a black-box
timestepper, we compute the coarse-grained equilibrium bifurcation diagrams,
examine the stability of the solution branches and perform a rare-events
analysis with respect to certain characteristics of the underlying network
topology such as the connectivity degree

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## Laws of rare events for deterministic and random dynamical systems

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

Relevância na Pesquisa

46.99793%

The object of this paper is twofold. From one side we study the dichotomy, in
terms of the Extremal Index of the possible Extreme Value Laws, when the rare
events are centred around periodic or non periodic points. Then we build a
general theory of Extreme Value Laws for randomly perturbed dynamical systems.
We also address, in both situations, the convergence of Rare Events Point
Processes. Decay of correlations against $L^1$ observables will play a central
role in our investigations.

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## An Efficient Self-optimized Sampling Method for Rare Events in Nonequilibrium Systems

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/08/2013
Português

Relevância na Pesquisa

46.99793%

Rare events such as nucleation processes are of ubiquitous importance in real
systems. The most popular method for nonequilibrium systems, forward flux
sampling (FFS), samples rare events by using interfaces to partition the whole
transition process into sequence of steps along an order parameter connecting
the initial and final states. FFS usually suffers from two main difficulties:
low computational efficiency due to bad interface locations and even being not
applicable when trapping into unknown intermediate metastable states. In the
present work, we propose an approach to overcome these difficulties, by
self-adaptively locating the interfaces on the fly in an optimized manner.
Contrary to the conventional FFS which set the interfaces with euqal distance
of the order parameter, our approach determines the interfaces with equal
transition probability which is shown to satisfy the optimization condition.
This is done by firstly running long local trajectories starting from the
current interface $\l_i$ to get the conditional probability distribution $P_c$,
and then determining $\l_{i+1}$ by equalling $P_c$ to a give value $p_0$. With
these optimized interfaces, FFS can be run in a much efficient way. In
addition, our approach can conveniently find the intermediate metastable states
by monitoring some special long trajectories that nither end at the initial
state nor reach the next interface...

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## Rare events statistics of random walks on networks: localization and other dynamical phase transitions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/06/2015
Português

Relevância na Pesquisa

47.205977%

Rare event statistics for random walks on complex networks are investigated
using the large deviations formalism. Within this formalism, rare events are
realized as typical events in a suitably deformed path-ensemble, and their
statistics can be studied in terms of spectral properties of a deformed Markov
transition matrix. We observe two different types of phase transition in such
systems: (i) rare events which are singled out for sufficiently large values of
the deformation parameter may correspond to {\em localized\/} modes of the
deformed transition matrix, (ii) "mode-switching transitions" may occur as the
deformation parameter is varied. Details depend on the nature of the observable
for which the rare event statistics is studied, as well as on the underlying
graph ensemble. In the present letter we report on the statistics of the
average degree of the nodes visited along a random walk trajectory in
Erd\H{o}s-R\'enyi networks. Large deviations rate functions and localization
properties are studied numerically. For observables of the type considered
here, we also derive an analytical approximation for the Legendre transform of
the large-deviations rate function, which is valid in the large connectivity
limit. It is found to agree well with simulations.; Comment: 5 pages...

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## Quick Search for Rare Events

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/10/2012
Português

Relevância na Pesquisa

47.29166%

Rare events can potentially occur in many applications. When manifested as
opportunities to be exploited, risks to be ameliorated, or certain features to
be extracted, such events become of paramount significance. Due to their
sporadic nature, the information-bearing signals associated with rare events
often lie in a large set of irrelevant signals and are not easily accessible.
This paper provides a statistical framework for detecting such events so that
an optimal balance between detection reliability and agility, as two opposing
performance measures, is established. The core component of this framework is a
sampling procedure that adaptively and quickly focuses the
information-gathering resources on the segments of the dataset that bear the
information pertinent to the rare events. Particular focus is placed on
Gaussian signals with the aim of detecting signals with rare mean and variance
values.

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## A probabilistic decomposition-synthesis method for the quantification of rare events due to internal instabilities

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/11/2015
Português

Relevância na Pesquisa

47.078174%

#Physics - Computational Physics#Nonlinear Sciences - Chaotic Dynamics#Physics - Data Analysis, Statistics and Probability

We consider the problem of probabilistic quantification of dynamical systems
that have heavy-tailed characteristics. These heavy-tailed features are
associated with rare transient responses due to the occurrence of internal
instabilities. Here we develop a computational method, a probabilistic
decomposition-synthesis technique, that takes into account the nature of
internal instabilities to inexpensively determine the non-Gaussian probability
density function for any arbitrary quantity of interest. Our approach relies on
the decomposition of the statistics into a `non-extreme core', typically
Gaussian, and a heavy-tailed component. This decomposition is in full
correspondence with a partition of the phase space into a `stable' region where
we have no internal instabilities, and a region where non-linear instabilities
lead to rare transitions with high probability. We quantify the statistics in
the stable region using a Gaussian approximation approach, while the
non-Gaussian distributions associated with the intermittently unstable regions
of the phase space are inexpensively computed through order-reduction methods
that take into account the strongly nonlinear character of the dynamics. The
probabilistic information in the two domains is analytically synthesized
through a total probability argument. The proposed approach allows for the
accurate quantification of non-Gaussian tails at more than 10 standard
deviations...

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## Reduced order prediction of rare events in unidirectional nonlinear water waves

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 20/01/2015
Português

Relevância na Pesquisa

47.151616%

We consider the problem of short-term prediction of rare, extreme water waves
in unidirectional fields, a critical topic for ocean structures and naval
operations. One possible mechanism for the occurrence of such rare,
unusually-intense waves is nonlinear wave focusing. Recent results have
demonstrated that random localizations of energy, induced by the dispersive
mixing of different harmonics, can grow significantly due to localized
nonlinear focusing. Here we show how the interplay between i) statistical
properties captured through linear information such as the waves power spectrum
and ii) nonlinear dynamical properties of focusing localized wave groups
defines a critical length scale associated with the formation of extreme
events. The energy that is locally concentrated over this length scale acts as
the "trigger" of nonlinear focusing for wave groups and the formation of
subsequent rare events. We use this property to develop inexpensive, short-term
predictors of large water waves. Specifically, we show that by merely tracking
the energy of the wave field over the critical length scale allows for the
robust, inexpensive prediction of the location of intense waves with a
prediction window of 25 wave periods. We demonstrate our results in numerical
experiments of unidirectional water wave fields described by the Modified
Nonlinear Schrodinger equation. The presented approach introduces a new
paradigm for understanding and predicting intermittent and localized events in
dynamical systems characterized by uncertainty and potentially strong nonlinear
mechanisms.

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## Dynamics of the Wang-Landau algorithm and complexity of rare events for the three-dimensional bimodal Ising spin glass

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/05/2004
Português

Relevância na Pesquisa

46.99793%

We investigate the performance of flat-histogram methods based on a
multicanonical ensemble and the Wang-Landau algorithm for the three-dimensional
+/- J spin glass by measuring round-trip times in the energy range between the
zero-temperature ground state and the state of highest energy. Strong
sample-to-sample variations are found for fixed system size and the
distribution of round-trip times follows a fat-tailed Frechet extremal value
distribution. Rare events in the fat tails of these distributions corresponding
to extremely slowly equilibrating spin glass realizations dominate the
calculations of statistical averages. While the typical round-trip time scales
exponential as expected for this NP-hard problem, we find that the average
round-trip time is no longer well-defined for systems with N >= 8^3 spins. We
relate the round-trip times for multicanonical sampling to intrinsic properties
of the energy landscape and compare with the numerical effort needed by the
genetic Cluster-Exact Approximation to calculate the exact ground state
energies. For systems with N >= 8^3 spins the simulation of these rare events
becomes increasingly hard. For N >= 14^3 there are samples where the
Wang-Landau algorithm fails to find the true ground state within reasonable
simulation times. We expect similar behavior for other algorithms based on
multicanonical sampling.; Comment: 9 pages...

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## Control of rare events in reaction and population systems by deterministic processes and the speedup of disease extinction

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/01/2011
Português

Relevância na Pesquisa

47.151616%

#Condensed Matter - Statistical Mechanics#Physics - Chemical Physics#Quantitative Biology - Populations and Evolution

We consider control of reaction and population systems by deterministically
imposed transitions between the states with different numbers of particles or
individuals. Even where the imposed transitions are significantly less frequent
than spontaneous transitions, they can exponentially strongly modify the rates
of rare events, including switching between metastable states or population
extinction. We also study optimal control of rare events, and specifically,
optimal control of disease extinction for a limited vaccine supply. A
comparison is made with control of rare events by modulating the rates of
elementary transitions rather than imposing transitions. It is found that,
unexpectedly, for the same mean control parameters, controlling the transitions
rates can be more efficient.

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## Distribution of Return Periods of Rare Events in Correlated Time Series

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

Relevância na Pesquisa

46.99793%

We study the effect on the distribution of return periods of rare events of
the presence in a time series of finite-term correlations with non-exponential
decay. Precisely, we analyze the auto-correlation function and the statistics
of the return intervals of extreme values of the resistance fluctuations
displayed by a resistor with granular structure in a nonequilibrium stationary
state. The resistance fluctuations, $\delta R$, are calculated by Monte Carlo
simulations using the SBRN model introduced some years ago by Pennetta,
Tref\'an and Reggiani and based on a resistor network approach. A rare event
occurs when $\delta R$ overcomes a threshold value $q$ significantly higher
than the average value of the resistance. We have found that for highly
disordered networks, when the auto-correlation function displays a
non-exponential decay but yet the resistance fluctuations are characterized by
a finite correlation time, the distribution of return intervals of the extreme
values is well described by a stretched exponential, with exponent largely
independent of the threshold $q$. We discuss this result and some of the main
open questions related to it, also in connection with very recent findings by
other authors concerning the observation of stretched exponential distributions
of return intervals of extreme events in long-term correlated time series.; Comment: 10 pages...

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## Can rare events explain the equity premium puzzle?

Fonte: Financial Markets Group, London School of Economics and Political Science
Publicador: Financial Markets Group, London School of Economics and Political Science

Tipo: Monograph; NonPeerReviewed
Formato: application/pdf

Publicado em /03/2008
Português

Relevância na Pesquisa

47.151616%

Probably not. First, allowing the probabilities attached to the states of the economy to differ from their sample frequencies, the Consumption-CAPM is still rejected by the data and requires a very high level of Relative Risk Aversion(RRA) in order to rationalize the stock market risk premium. This result holds for a variety of data sources and samples –including ones starting as far back as 1890. Second, we elicit the likelihood of observing an Equity Premium Puzzle (EPP) if the data were generated by the rare events probability distribution needed to rationalize the puzzle with a low level of RRA. We find that the historically observed EPP would be very unlikely to arise. Third, we find that the rare events explanation of the EPP signi…cantly worsens the ability of the Consumption-CAPM to explain the cross-section of asset returns. This is due to the fact that, by assigning higher probabilities to bad –economy wide –states in which consumption growth is low and all the assets in the cross-section tend to yield low returns, the rare events hypothesis reduces the cross-sectional dis-persion of consumption risk relative to the cross-sectional variation of average returns

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