Unlike small molecule drug products, biological products are therapeutic agents producted using of a living system or organism. Thus, the development of biologic products is a very different and complicated process that is sensitive to environmental factors such as light and temperature. Therefore, the therapeutic effect of follow-on biologic products may not be equivalent to the innovative products even though the average biosimilarity has been established. Thus, Chow et al. suggested that the assessment of biosimilarity between biologic products should be conducted on the basis of variability in 2010. In this article, we propose a biosimilar index that is derived on the basis of estimated reproducibility probability approach and Bayesian approach, respectively. We conducted simulation studies to empirically investigate the relationship of reproducibility probability under various parameter combinations. The simulation results demonstrate that the proposed method based on biosimilar index can reflect the characteristics and impact of variability on the therapeutic effect of biologic products. Copyright © 2012 John Wiley & Sons, Ltd.