Biases in the assessment of colorectal cancer screening programmes using hemoccult.

Many reports of screening programmes using the 'Hemoccult' faecal occult blood test have been published recently. Falsely optimistic results in such short term or nonexperimental studies can be caused by three biases of method: the lead-time bias, the length bias, and the volunteer bias. Their mechanisms, and methods for counteracting their effects are described. Literature biases that occur in papers written on this subject by enthusiasts are also described. Many apparently valuable screening tests have failed in the practical setting, so caution in the use of occult blood screening is urged until long term follow-up results are obtained from a trial currently being conducted in Minnesota.