Stage migration (Will Rogers phenomenon)
- Bias
Better staging moves patients to a more advanced stage: both stages seem to improve although nobody lives longer.
What it is
Stage migration happens when a new diagnostic technique reveals spread that could not be seen before, and some patients who would have been classed in an early stage move to an advanced one. Those who migrate are the worst-prognosis patients of the early stage but the best-prognosis patients of the advanced stage, so survival in each stage rises while overall survival does not change. It is called the Will Rogers phenomenon and it affects comparisons of stage-specific survival between eras or centres.
How it arises
- Before: the early stage includes patients with hidden disease who live less than the rest of the group.
- After: those patients move to the advanced stage, where they live longer than the group's average.
- The average of both groups rises; the total number of deaths does not change.
Worked example
- 200 patients, 5-year survival. Before: stage I 100 (80 alive, 80%); stage II 100 (40 alive, 40%).
- Overall: 120 of 200 = 60%. A new, more sensitive imaging test arrives.
- It finds hidden metastases in 20 stage I patients, of whom 10 live 5 years (50%).
- New stage I: 80 patients and 70 alive = 87.5% (before 80%).
- New stage II: 120 patients and 50 alive = 41.7% (before 40%).
- Overall: 120 of 200 = 60%, unchanged. Reading: both stages improve and nobody lives longer.
5-year survival before and after reclassification (example)
| Group | Before | After |
|---|---|---|
| Stage I | 80/100 = 80% | 70/80 = 87.5% |
| Stage II | 40/100 = 40% | 50/120 = 41.7% |
| Total | 120/200 = 60% | 120/200 = 60% |
Markers
- Will Rogers
- Stage
- TNM
How to prevent or control it
- Compare eras or centres with overall survival or mortality, not only by stage.
- If comparing by stage, reclassify everyone with the same criteria and the same tests.
- In trials, require the same staging work-up in every arm.
Common pitfalls
- Crediting a new treatment for the stage-specific improvement of a recent series against a historical one.
- Comparing stage-specific results of a hospital with more advanced imaging against one with less.
- Confusing it with lead-time bias: here the time of diagnosis does not change, the classification does.
Test yourself
Think of the answer, then tap the question to see it.
Why does survival improve in both stages with stage migration?
Those who migrate are the worst of the early stage and the best of the advanced stage, so each group's average rises.
Which measure does stage migration leave unchanged?
The overall survival (or mortality) of all patients together.
A 2025 series with PET against a 2005 series without it shows better survival in every stage. What do you suspect?
Stage migration; compare overall survival or reclassify with the same criteria.
Elsewhere in DurgiPedia
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: SEER Cancer Statistics Review 1975-2016: Introduction (National Cancer Institute, 2019); Measures of Cancer Survival (National Cancer Institute, Surveillance Research Program, 2026).
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Representative model, for educational purposes only.