data(Oph)The data set consists of two groups of participants: (1) the experimental group (i.e., group E in which participants receiving 15,000 IU of vitamin A per day) and (2) the control group (i.e., group C in which participants receiving 75 IU of vitamin A per day).
The participants were enrolled over a 2-year period (1984-1987) and followed for a maximum of 6 years. The follow-up was terminated in September 1991. Some participants dropped out of the study before September 1991 and had not failed. Dropouts were due to death, other diseases, or side effects possibly due to the study medications, or unwillingness to comply (take study medications). There are 6 time points (at 1st year, 2nd year, 3rd year, 4th year, 5-th year, and 6-th year) in this data set.
Rosner (2006, page 786) defined the participants who do not reach a disease endpoint during their period of follow-up as censored observations. A participant has been censored at time $t$ if the participant has been followed up to time $t$ and has not failed. Noninformative censoring is assumed. That is, participants who are censored have the same underlying survival curve after their censoring time as patients who are not censored.
Rosner B. (2006). Fundamentals of Biostatistics. (6-th edition). Thomson Brooks/Cole.