Nonparametric Maximum Likelihood Estimation by the Method of Sieves

Maximum likelihood estimation often fails when the parameter takes values in an infinite dimensional space. For example, the maximum likelihood method cannot be applied to the completely nonparametric estimation of a density function from an iid sample; the maximum of the likelihood is not attained by any density. In this example, as in many other examples, the parameter space (positive functions with area one) is too big. But the likelihood method can often be salvaged if we first maximize over a constrained subspace of the parameter space and then relax the constraint as the sample size grows. This is Grenander's "method of sieves." Application of the method sometimes leads to new estimators for familiar problems, or to a new motivation for an already well-studied technique. We will establish some general consistency results for the method, and then we will focus on three applications.