Population size plays a major role in determining the performance of any biometric authentication system, particularly when such systems are used for civilian applications. In this paper, we propose to improve the performance of a biometric authentication system by modeling the variation in the population participating in the process. We show that this technique is helpful when the number of users enrolled into the system is very large as compared to the number of users which actually participate in the process. We show results on aperiodically and periodically varying population using Markov models.