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An analysis of survival differences between clinically and screen-detected cancer patients.

Survival differences between clinically and screen-detected cancer patients partly result from biases. Well known are lead-time, length bias and overdiagnosis. The survival of the clinically detected patients in the study group of the HIP breast cancer screening project is corrected for these biases. The resulting survival curve is only slightly worse than the survival of the screen-detected patients. This suggests a very modest mortality reduction by screening. A much larger reduction is obtained from an analysis of the complete HIP results, including those of the control group. It is concluded that a large unexpected selection bias is present. This bias would not have been detected if the HIP study had not contained a randomized control group. A misleading and pessimistic conclusion on the effectiveness of breast cancer screening would thus have resulted. This conclusion reinforces the need for randomized studies.