'Length-time bias' in cancer screening programmes refers to:
- A Patients diagnosed by screening appearing to survive longer only because diagnosis is made earlier, not because treatment prolonged life
- B Screening selectively detecting slow-growing, less lethal cancers while missing fast-growing aggressive ones ✓
- C Volunteers for screening being healthier than the general population
- D Overdiagnosis of cancers that would never cause clinical disease
Correct answer: B. Screening selectively detecting slow-growing, less lethal cancers while missing fast-growing aggressive ones
Explanation
Length-time bias (length-biased sampling) occurs because rapidly growing tumors spend less time in the presymptomatic detectable phase; periodic screening therefore preferentially detects slow-growing, less lethal tumors with longer sojourn times, making screening appear more effective than it truly is. Lead-time bias is the earlier-diagnosis-appearing-longer-survival phenomenon in option A.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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