Evidence is available from many different kinds of surveys that repeated interviewing of the same persons can frequently change response patterns. For many characteristics, estimates from different panels relating to the same time period do not have the same expected value. In panel surveys, estimation techniques frequently take advantage of the correlation between observations on identical persons over time. The existence of a bias, its effect on both ratio estimates and composite estimates, and a comparison of the estimated mean-square errors of ratio and composite estimates are illustrated with data from the Current Population Survey.