Figures are from-- and longer (but not necessarily clearer) explanations can be found here: http://ecp.acponline.org/marapr99/primer.htm
Lead-time bias: Overestimation of survival duration due to by screening than clinical presentation.
Length-time bias: Overestimation of survival duration due to the relative excess of cases that are slowly progressing.
- Imagine all 12 cases below are the same disease. Instead of being seen as rapidly progressive 50% of the time, rapidly progressive disease is only detected in 33% of positive screens.
Length-time bias is due to slow cases being detected more often simply because they are slowly progressing. Remember the "g" in length is for slowly proressin.
Lead-time bias is due to early detection. Remember the "d" in lead is for early detection.