Survivorship analysis, which is often encountered in the medical
literature, is used to calculate the probability of a certain event, such
as failure of a prosthesis, as a function of the time elapsed since an
operation. Possible pitfalls in the use of this method are related to the
size of the population of patients and the definition of how the outcome is
measured. We studied the outcomes of 204 total knee arthroplasties in 165
patients, using six different end-points, in order to illustrate these
problems. Survivorship estimates that are cited without confidence
intervals have little clinical value.