This paper investigates inter-tablet coating variability, specifically, the variability of tablet residence times within the spray zone of a horizontal coating pan. Results from experiments, discrete element method (DEM) computer simulations, and an analytical model developed to describe the coating mass distribution are presented. The simulations indicate that the coefficient of variation of tablet residence times, and subsequently, of coating mass, decreases with time following a power law relation. The theoretical model demonstrates that the coefficient of variation of residence time for a randomly mixed tablet bed is inversely proportional to the square root of the number of coating "trials". DEM simulations show that during each pan revolution, tablets in the spray zone remain in a quasi-segregated state from tablets located outside the spray zone for some time period termed Delta t(seg). Increasing the pan's Froude number, the spray zone aspect ratio, and the tablet-tablet and tablet-pan friction coefficient all act to decrease Delta t(seg), leading to more uniform residence times and less inter-tablet coating variability for a given operating time. The relationship between Delta t(seg), and tablet load is more complex due changes in bed dynamics. In addition to the variability studies, a model is developed that relates coating fraction, effective mass now rate, Delta t(seg), and the desired coating mass to the allowable fraction of tablets with a coating mass lying outside of a specified range of coating masses. (C) 2009 Elsevier Ltd. All rights reserved.