A discrete-time mover-stayer (MS) model is an extension of a discrete-time Markov chain, which assumes a simple form of population heterogeneity. The individuals in the population are either stayers, who never leave their initial states or movers who move according to a Markov chain. We, in turn, propose an extension of the MS model by specifying the stayer's probability as a logistic function of an individual's covariates. Such extension has been recently discussed for a continuous time MS but has not been considered before for a discrete time one. This extension allows for an in-sample classification of subjects who never left their initial states into stayers or movers. The parameters of an extended MS model are estimated using the expectation-maximization algorithm. A novel bootstrap procedure is proposed for out of sample validation of the in-sample classification. The bootstrap procedure is also applied to validate the in-sample classification with respect to a more general dichotomy than the MS one. The developed methods are illustrated with the data set on installment loans. But they can be applied more broadly in credit risk area, where prediction of creditworthiness of a loan borrower or lessee is of major interest. KEYWORDS bootstrap, covariate effects, credit risk, discrete-time mover-stayer model, EM algorithm, status classification
INTRODUCTIONA discrete-time mover-stayer (MS) model is an extension of a discrete-time Markov chain. It assumes a simple form of population heterogeneity. There are 2 types of individuals in a population: stayers and movers. Stayers never leave their initial states, and movers move among the states according to a discrete-time Markov chain. In one of the first applications of stochastic processes to social sciences, Blumen et al 1 introduced the MS model to study industrial mobility in the United States. The study assumed that in a given industry, there is some unknown proportion of workers who never leave it and that the remaining proportion of workers move among industries according to a Markov chain. But the estimators of the MS model's parameters-the proportions of stayers in each state and the transition matrix of the movers obtained in the study from a sample of independent realizations of the model-were inconsistent.Goodman 2 provided consistent estimators of these parameters. Frydman 3 obtained the maximum likelihood estimators (mles) of the MS model's parameters by direct maximization of the observed likelihood function, and Fuchs and Greenhouse 4 did so by using the expectation-maximization (EM) algorithm. In the MS model described above, the probability of being a stayer in a given state is the same for all individuals and is equal to the proportion of stayers in that state. In this paper, we propose an extension of the discrete-time MS model, which 196