Uniformly Efficient Importance Sampling for the Tail Distribution of Sums of Random Variables

  • Authors:
  • Paul Glasserman;Sandeep Juneja

  • Affiliations:
  • Columbia University, New York, New York 10027;Tata Institute of Fundamental Research, Mumbai, India 400005

  • Venue:
  • Mathematics of Operations Research
  • Year:
  • 2008

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Abstract

Successful efficient rare-event simulation typically involves using importance sampling tailored to a specific rare event. However, in applications one may be interested in simultaneous estimation of many probabilities or even an entire distribution. In this paper, we address this issue in a simple but fundamental setting. Specifically, we consider the problem of efficient estimation of the probabilities P(Sn ≥ na) for large n, for all a lying in an interval A, where Sn denotes the sum of n independent, identically distributed light-tailed random variables. Importance sampling based on exponential twisting is known to produce asymptotically efficient estimates when A reduces to a single point. We show, however, that this procedure fails to be asymptotically efficient throughout A when A contains more than one point. We analyze the best performance that can be achieved using a discrete mixture of exponentially twisted distributions, and then present a method using a continuous mixture. We show that a continuous mixture of exponentially twisted probabilities and a discrete mixture with a sufficiently large number of components produce asymptotically efficient estimates for all a ∈ A simultaneously.