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531f10MH1

# 531f10MH1 - STAT 531 Metropolis-Hastings(MH Algorithm HM...

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STAT 531: Metropolis-Hastings (MH) Algorithm HM Kim Department of Mathematics and Statistics University of Calgary Fall 2010 1/24

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One problem with applying Monte Carlo integration is in obtaining samples from some complex probability distribution p ( x ). Attempts to solve this problem are the roots of MCMC methods. Fall 2010 2/24
# Recall: Monte Carlo integration on the Markov Chain Subject to regularly conditions, the chain will gradually “forget” its initial state and the chain will eventually converges to a unique stationary distribution. We denote the unique stationary distribution by f , and as n increases, the sampled point X ( t ) will look increasingly like dependent samples from f . The standard average, 1 T T t =1 h ( X ( t ) ) E f [ h ( X )] that lies at the basis of Monte Carlo method can be applied in Markov Chain Monte Carlo (MCMC) settings. One problem: our draws are not independent, which we required for Monte Carlo Integration to work (remember SLLN). Luckily, we have the Ergodic Theorem . Fall 2010 3/24

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Suppose our goal is to draw samples from some distribution p ( θ ), where p ( θ ) = f ( θ ) K , where the normalizing constant K may not be known, and very difficult to compute. The Metropolis algorithm generates a sequence of draws from this distribution is as follows: 1. start with any initial value θ 0 satisfying f ( θ 0 ) > 0 2. using current θ value, sample a candidate point θ * from some jumping distribution q ( θ 2 | θ 1 ) (proposal distribution), which is the probability of returning a value of θ 2 given a previous value of θ 1 . (the restriction on jump density is that it is symmetric, i.e. q ( θ 2 | θ 1 ) = q ( θ 1 | θ 2 )) Fall 2010 4/24
3. given the candidate point θ * , calculate the ratio of the density at the candidate ( θ * ) and current ( θ t - 1 ) points, α = p ( θ * ) p ( θ t - 1 ) = f ( θ * ) f ( θ t - 1 ) 4. if the jump increases the density ( α > 1), accept the candidate point (set θ t = θ * ) and return to step 2 if the jump decreases the density ( α < 1), then with probability α accept the candidate point, else reject it and return to step 2 # summarize the Metropolis sampling((3) and (4)) as first computing ρ = min f ( θ * ) f

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531f10MH1 - STAT 531 Metropolis-Hastings(MH Algorithm HM...

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