Censored observations: An observation xi on some variable of interest is consired to be censored if it is known that xi Li (left-censored)or xi Ui (right-censored) where Li and Ui are the ?xed values. Such observations arise most often in studies where the main response variable is time until a particular event occurs (for instance, time to death) when at the completion of the study, the event of the interest has not happened to many subjects.