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"Frontmatter". In: Analysis of Financial Time Series

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430 MCMC METHODS(a) GARCH with time-varying correlationvol20 40 60 80 120 1601970 1980 1990 2000year(b) Stochastic volatilityvol20 40 60 80 120 1601970 1980 1990 2000yearFigure 10.8. <strong>Time</strong> plots <strong>of</strong> fitted volatilities for monthly log returns <strong>of</strong> IBM stock from 1962to 1999: (a) a GARCH model with time-varying correlations, and (b) a bivariate stochasticvolatility model estimated by Gibbs sampling with 300 + 1000 iterations.Tsay (1994) discuss a Gibbs sampling procedure to estimate such a model when thevolatility in each state is constant over time. These authors applied the procedureto estimate a Markov switching model with different dynamics and mean levels fordifferent states to the quarterly growth rate <strong>of</strong> U.S. real gross national product, seasonallyadjusted, and obtained some interesting results. For instance, the dynamics<strong>of</strong> the growth rate are significantly different between periods <strong>of</strong> economic “contraction”and “expansion.” Since this chapter is concerned with asset returns, we focuson models with volatility switching.Suppose that an asset return r t follows a simple two-state Markov switchingmodel with different risk premiums and different GARCH dynamics:r t ={β1√ht + √ h t ɛ t , h t = α 10 + α 11 h t−1 + α 12 a 2 t−1if s t = 1β 2√ht + √ h t ɛ t , h t = α 20 + α 21 h t−1 + α 22 a 2 t−1if s t = 2,(10.40)where a t = √ h t ɛ t , {ɛ t } is a sequence <strong>of</strong> Gaussian white noises with mean zeroand variance 1, and the parameters α ij satisfy some regularity conditions so thatthe unconditional variance <strong>of</strong> a t exists. The probability transition from one state to

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