In this research, the Cox-proportional hazard model is used to investigate the effect of various values of vitamin A (3000, 4500 and 6000 IU kg(-1)) and sesame oil on the passive avoidance memory of rats by shuttle box. Present results confirm that various values of vitamin A do not improve the passive avoidance memory of rats (p < 0.05). We found that the animals are clustered (p < 0.001) and applying shared log-normal frailty for clustering improves present results (p < 0.05) such that sesame oil improves the passive avoidance memory task (p < 0.05). Therefore we should consider clustering in the analysis of biological data or we should use cloned animals.