48,151 research outputs found
Maximum likelihood estimation for social network dynamics
A model for network panel data is discussed, based on the assumption that the
observed data are discrete observations of a continuous-time Markov process on
the space of all directed graphs on a given node set, in which changes in tie
variables are independent conditional on the current graph. The model for tie
changes is parametric and designed for applications to social network analysis,
where the network dynamics can be interpreted as being generated by choices
made by the social actors represented by the nodes of the graph. An algorithm
for calculating the Maximum Likelihood estimator is presented, based on data
augmentation and stochastic approximation. An application to an evolving
friendship network is given and a small simulation study is presented which
suggests that for small data sets the Maximum Likelihood estimator is more
efficient than the earlier proposed Method of Moments estimator.Comment: Published in at http://dx.doi.org/10.1214/09-AOAS313 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Volatility forecasting
Volatility has been one of the most active and successful areas of research in time series econometrics and economic forecasting in recent decades. This chapter provides a selective survey of the most important theoretical developments and empirical insights to emerge from this burgeoning literature, with a distinct focus on forecasting applications. Volatility is inherently latent, and Section 1 begins with a brief intuitive account of various key volatility concepts. Section 2 then discusses a series of different economic situations in which volatility plays a crucial role, ranging from the use of volatility forecasts in portfolio allocation to density forecasting in risk management. Sections 3, 4 and 5 present a variety of alternative procedures for univariate volatility modeling and forecasting based on the GARCH, stochastic volatility and realized volatility paradigms, respectively. Section 6 extends the discussion to the multivariate problem of forecasting conditional covariances and correlations, and Section 7 discusses volatility forecast evaluation methods in both univariate and multivariate cases. Section 8 concludes briefly. JEL Klassifikation: C10, C53, G1
Modeling long-range memory with stationary Markovian processes
In this paper we give explicit examples of power-law correlated stationary
Markovian processes y(t) where the stationary pdf shows tails which are
gaussian or exponential. These processes are obtained by simply performing a
coordinate transformation of a specific power-law correlated additive process
x(t), already known in the literature, whose pdf shows power-law tails 1/x^a.
We give analytical and numerical evidence that although the new processes (i)
are Markovian and (ii) have gaussian or exponential tails their autocorrelation
function still shows a power-law decay =1/T^b where b grows with a
with a law which is compatible with b=a/2-c, where c is a numerical constant.
When a<2(1+c) the process y(t), although Markovian, is long-range correlated.
Our results help in clarifying that even in the context of Markovian processes
long-range dependencies are not necessarily associated to the occurrence of
extreme events. Moreover, our results can be relevant in the modeling of
complex systems with long memory. In fact, we provide simple processes
associated to Langevin equations thus showing that long-memory effects can be
modeled in the context of continuous time stationary Markovian processes.Comment: 5 figure
Exact solution of diffusion limited aggregation in a narrow cylindrical geometry
The diffusion limited aggregation model (DLA) and the more general dielectric
breakdown model (DBM) are solved exactly in a two dimensional cylindrical
geometry with periodic boundary conditions of width 2. Our approach follows the
exact evolution of the growing interface, using the evolution matrix E, which
is a temporal transfer matrix. The eigenvector of this matrix with an
eigenvalue of one represents the system's steady state. This yields an estimate
of the fractal dimension for DLA, which is in good agreement with simulations.
The same technique is used to calculate the fractal dimension for various
values of eta in the more general DBM model. Our exact results are very close
to the approximate results found by the fixed scale transformation approach.Comment: 18 pages RevTex, 6 eps figure
On a flexible construction of a negative binomial model
This work presents a construction of stationary Markov models with
negative-binomial marginal distributions. A simple closed form expression for
the corresponding transition probabilities is given, linking the proposal to
well-known classes of birth and death processes and thus revealing interesting
characterizations. The advantage of having such closed form expressions is
tested on simulated and real data.Comment: Forthcoming in "Statistics & Probability Letters
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