Transcription of Chapter 6 Importance sampling
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Chapter 6 Importance The basicsTo movtivate our discussion consider the following situation. We want to use monte Carlo tocompute =E[X]. There is an eventEsuch thatP(E) is small butXis small outside we run the usual monte Carlo algorithm the vast majorityof our samples ofXwill beoutsideE. But outside ofE,Xis close to zero. Only rarely will we get a sample inEwhereXis not of the time we think of our problem as trying to compute the mean of some randomvariableX. For Importance sampling we need a little more structure. Weassume that therandom variable we want to compute the mean of is of the formf(~X) where~Xis a randomvector. We will assume that the joint distribution of~Xis absolutely continous and letp(~x) bethe density.
So we do a Monte Carlo simulation of Eq[e−X(1−α)] where X has distribution q. Note that e−X(1−α) is a bounded random variable. The second general idea we illustrate involves rare-event simulation. This refers to the situation where you want to compute the probabily of an event when that probability is very small.
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