The assessment of lead time and length bias in the evaluation of screening programmes.

The evaluation of screening programmes for cancer is discussed with particular reference to lead time and length bias. The epidemiological approach, concentrating on the population screened instead of the cases detected, leads to a useful evaluation of the concepts of lead time and length bias, being closely related to the natural history of the disease in question. This is shown by reviewing screening programmes for these different diseases: breast cancer, lung cancer and cervical cancer.