Introduction to Kaplan and Meier (1958) Nonparametric Estimation from Incomplete Observations

During the late 1940s and early 1950s, stimulated in part by Sir Bradford Hill’s (1951) advocacy of the randomized clinical trial as a cornerstone of modern scientific medicine, medical research workers began to undertake numerous follow-up studies of patient populations in order to assess the effectiveness of medical treatment. A challenging problem in the analysis of data from such investigations, whose typical endpoint was the death of a patient, stemmed from the fortunate fact that not all the patients had died at the end of the study. Thus, some of the observed lifetimes were censored on the right, meaning simply that they were known only to exceed an observation limit equal to the time interval from the patient’s entry on study to its close or, if follow-up was incomplete, to the time of loss from further observation. This precluded the use of classical statistical procedures based on averages.