Stage bias refers to a systematic distortion in study results that occurs when comparing outcomes between groups with different stages of disease, especially in cancer or chronic illness research.

It typically arises when:

• One group is diagnosed earlier (e.g., due to screening) and appears to have better outcomes, not because the treatment is more effective, but because the disease is detected at an earlier, less advanced stage.
• Comparisons are made without adjusting for disease stage at diagnosis, leading to confounding.

Patients undergoing regular screening may be diagnosed at an earlier stage and seem to live longer, even if their actual time of death is unchanged. This can overestimate the benefits of screening or early intervention.

• Lead Time Bias
• Length Time Bias
• Confounding

Understanding stage bias is essential for the correct interpretation of:

• Screening program effectiveness
• Survival statistics
• Comparisons between treated and untreated groups