Lead time bias
Occurs when outcomes such as survival, as measured from the time of diagnosis, may be increased not because patients live longer, but because screening lengthens the time that they know they have disease.
e.g breast cancer was found in group A using a screeng test, at mean age of 30 yrs n death occured at mean age 60....Length time bias
breast cancer was found in group B using clincal symtoms, at mean age of 45 yrs, and death occured at mean age 60
It was concluded that screening improved survival after dx, which is wrong. death occured at same time...only now pts know that they have cancer early...
solution: use life expectancy rather than survival after dx.
Occurs when patients whose disease is discovered by screening also may appear to do better, or live longer, than people whose disease presents clinically with symptoms. Screening tends to detect disease that is destined to progress slowly and, therefore, has a good prognosis
e.g group A screened for all cancers
group b not screened
later when death occured, autopsy was performed to find all cancer deaths in both groups...cancer pts in group A, who were screened, were concluded to live longer, as compared to those who were not screened.
the bias is that those who could not be screened had aggressive disease, developed early n died quickly...whereas those diagnosed by screening had slower disease, thats y they were diagnosed and had better survival. Screening actually had no impact on improvement in survivval.