Box 34 Number of patients by year and stage at diagnosis 3year survival rate in brackets

Period

Stage I

Stage II-IV

All patients

1980s 1990s

800 (95%) 600 (99%)

1000 (90%)

The overall survival rate can be calculated, approximately, as a weighted average ofthe survival rates for stage I and stage > I patients as follows:

In both cases, overall survival remains at 90 per cent. However, it is clear that a comparison of stage I patients treated in the 1980s with stage I patients treated perhaps differently in the 1990s would identify a difference in survival which could be attributed erroneously to changing treatments.

This illustrates a phenomenon known as stage migration, often referred to as the 'Will Rogers phenomenon' [10]. Will Rogers was an American humourist famously reported as saying of the 1930s Gold Rush, 'when the Okies moved from Oklahoma to California, they raised the IQ in both states,' implying that even the least intelligent people in Oklahoma were more intelligent than the typical Californian.

This example serves to illustrate a real difficulty in interpreting the results of treatment comparisons involving historical controls, and one in which adjustment for prognostic factors can do nothing to help.

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