A qualifier Q for the survival function to describe the prevalence of a transient condition

We propose an estimator of the prevalence of a transient condition among surviving patients using right censored data. The prevalence of opportunistic infection among surviving AIDS patients and the probability of being in tumour response following cancer therapy conditional on being alive are two examples of such functions. In essence these functions describe a major aspect of the quality of life for surviving patients and may be useful when viewed in conjunction with the survival curves themselves. The method is illustrated using data from a randomized trial of bone marrow transplant patients where the prevalence of chronic graft-versus-host disease is of interest. The non-parametric estimator which we have proposed is contrasted with estimators derived from Markov and semi-Markov models.