Pakistan Journal of Biological Sciences1028-88801812-5735Asian Network for Scientific Information10.3923/pjbs.2008.1263.1267 Reza ZadkaramiMohammad 92008119In 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
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