RAND Statistics Seminar Series

Effect of Length Biased Sampled Sojourn Times on the Survival Distribution from Screen-Detected Diseases

Presented by Karen Kafadar
University of Colorado at Denver
Thursday, April 15, 2004, 4:00 pm
Main Conference Room


Screen-detected cases form a length-biased sample among all cases of disease, since longer sojourn times afford more opportunity to be screen-detected. In contrast to the usual length-biased sampling situation, however, the length-biased sojourns (preclinical durations) are never observed, but their subsequent clinical durations are. We investigate the effect of length-biased sampling of sojourn times on the distribution (mainly, the mean and variance) of the observed clinical durations.

We show that, when preclinical and clinical durations are positively correlated, the mean clinical duration can be substantially inflated — even in the absence of any benefit on survival from the screening procedure. Consequently, the mean survival among cases in the screen-detected arm of a randomized screening trial will be longer than that among interval cases or cases that arise in the control arm, simply because of the length-bias phenomenon. We briefly discuss issues related to estimating the inflation.

(This work was performed in collaboration with Philip C. Prorok while Dr. Kafadar was a Guest Researcher in the Biometry Branch.)