The<unk>  a<unk>  in medical studies associated with how long a tumor takes to grow. Faster-growing tumors are more likely to be missed than slower ones, and have a poorer prognosis. Since slower-growing tumors are more likely to be detected, this presents a<unk>  in the resulting data that estimates a better prognosis than the actual prognosis.
Length time bias
Cognitive bias
Bias of an estimator
Unblinding
Mass effect 
Lotus effect
Confounding
Random variable