Method of Moments Using Monte Carlo Simulation

We present a computational approach to the method of moments using Monte Carlo simulation. Simple algebraic identities are used so that all computations can be performed directly using simulation draws and computation of the derivative of the log-likelihood. We present a simple implementation using the Newton-Raphson algorithm, with the understanding that other optimization methods may be used in more complicated problems. The method can be applied to families of distributions with unknown normalizing constants and can be extended to least-squares tting in the case that the number of moments observed exceeds the number of parameters in the model. The method can be further generalized to allow \moments" that are any function of data and parameters, including as a special case maximum likelihood for models with unknown normalizing constants or missing data. In addition to being used for estimation, our method may be useful for setting the parameters of a Bayes prior distribution by specifying moments of a distribution using prior information. We present two examples: speci cation of a multivariate prior distribution in a constrained-parameter family and estimation of parameters in an image model. The former example, used for an application in pharmacokinetics, motivated this work. This work is similar to a method of Ruppert (1985) in stochastic approximation, combines Monte Carlo simulation and the Newton-Raphson algorithm as in Penttinen (1984), uses computational ideas and importance sampling identities of Geyer and Thompson (1992), Gelfand and Carlin (1994), and Geyer (1994a) developed for Monte Carlo maximum likelihood, and has some similarities to the maximum likelihood methods of Wei and Tanner (1990).