Abstract        Survival analysis encompasses investigation of time to event data in clinical research. In survival analysis, it is common to have truncated data due to limited time span of the study or drop out of the subject for various reasons. The estimation of survivor function under left truncation was first discussed by Kaplan Meier by extending the well known product limit estimator of the survivor function. Later several parametric models are used for modelling survival data. The focus of this paper is to compare the models and their application in the study on increased access to emergency contraception by Raymond, E. et al. (2006) and their adverse impact on risk of sexual transmitted infections. Various models are computed using SAS (Statistical Analysis System) program. The findings are presented and discussed.

Keywords: emergency contraception.