Slide 1: Length time and lead time bias

Slide 2: What types of bias can occur when determining whether a cancer screening test reduces mortality?

Slide 3: Consider this case: SK Reen is a 65-year-old male whose friend was recently diagnosed with aggressive prostate cancer. He asks you whether there are any biochemical tests that could help detect cancer and help him live longer.

Slide 4: Length time bias occurs when screening is more likely to detect slow-growing disease that has a long phase without symptoms. Without screening, all cases (n=8) detected by symptoms only. Average survival time of detected cancers: 1.2 years. With screening, all cases (n=8) detected by screening. Average survival time of detected cancers: 6.8 years. Note the perceived survival time difference! There will appear to be a survival benefit to screening even when early detection does not improve outcomes!

Slide 5: Lead time boas occurs when patients diagnoses earlier appear to live longer because they know they have the disease for longer. In other words, awareness of the disease may make it falsely seem like early diagnoses patients live longer.

Slide 6: You explain to Mr. Reen that thought PSA screening could detect prostate cancer, it is more likely to find a slow growing tumor that would not have killed him in the first place (length time bias).

Furthermore, thought patients who are diagnosed by screening may appear to live longer, that is only because they were diagnosed earlier in the disease process (lead time bias).

Slide 7: How can studies mitigate length and lead time bias? Include all outcomes regardless of method of detection (symptoms vs. screening). Verify findings from observation studies with RCTs showing disease specific mortality benefit for early detection and treatment. Compare overall mortality in screened vs. unscreened individuals rather than survival from screening.


  • Evaluating Screening Programs. Link.
  • McCaffery K, Jacklyn G, Barratt A, Brodersen J, Glasziou P, Carter SM, Hicks NR, Howard K, Irwig L. Recommendations About Screening. In: Guyatt G, Rennie D, Meade MO, Cook DJ. eds. Users’ Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice, 3rd ed. McGraw Hill; 2015. Link.

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