Practice of Epidemiology Correcting for Lead Time and Length Bias in Estimating the Effect of Screen Detection on Cancer Survival

Determination of survival time among persons with screen-detected cancer is subject to lead time and length biases. The authors propose a simple correction for lead time, assuming an exponential distribution of the pre-clinical screen-detectable period. Assuming two latent categories of tumors, one of which is more prone to screen detection and correspondingly less prone to death from the cancer in question, the authors have developed a strategy of sensitivity analysis for various magnitudes of length bias. Here they demonstrate these methods using a series of 25,962 breast cancer cases (1988–2004) from the West Midlands, United Kingdom. bias (epidemiology); breast neoplasms; mass screening; models, statistical; survival Abbreviation: CI, confidence interval. In disease screening, the concepts of lead time and length bias have been familiar for decades (1, 2). Lead time is the amount of time by which the diagnosis has been advanced by screening. In analysis of survival from diagnosis, it con-stitutes an artificial addition to the survival time of screen-detected cases. Length bias is the phenomenon whereby more slowly growing tumors, with less capacity to prov