We define the notion of ‘importance’ of prognostic factors in studies of survival and suggest quantifying it by the Schemper–Henderson measure of explained variation. Conceptual differences to the standard approach for the statistical analysis of oncologic studies of survival are discussed and exemplified by means of a study of ovarian cancer. Explained variation permits to establish a ranking of the importance of factors, also if measured on different scales, or of different types (dichotomous, qualitative or continuous), and permits to compare groups of related factors. In practice, the importance of prognostic factors often is disappointingly low. From this, it follows that even strong and highly significant prognostic factors often do not translate into close determination of individual survival of patients.