==== Stage Bias ====

**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**.

=== Example ===
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.

=== Related Concepts ===
  * [[lead_time_bias|Lead Time Bias]]
  * [[length_time_bias|Length Time Bias]]
  * [[confounding|Confounding]]

=== Relevance ===
Understanding stage bias is essential for the correct interpretation of:
  * Screening program effectiveness
  * Survival statistics
  * Comparisons between treated and untreated groups