Cancer survival rates

An important point (and a nice piece of statistics writing for the general public) by Paul Krugman:

Beyond that, there’s a well-known problem with survival-rate comparisons, acknowledged in the Lancet Oncology study:

Cancer survival is a valuable indicator for international comparison of progress in cancer control,despite the fact that part of the variation in cancer survival identified in this study could be attributable to differences in the intensity of diagnostic activity (case-finding) in participating populations.

Here’s how I understand the over-diagnosis issue, in terms of an extreme example: suppose that there’s a form of cancer that kills people 7 years after it starts, and that there is in fact nothing you can do about it. Suppose that country A screens for cancer very aggressively, and always catches this cancer in year 1, while country B chooses to invest its medical resources differently, and never catches the cancer until year 4. In that case, country A will have a 100% 5-year survival rate, while country B will have a 0% 5-year survival rate — because survival is measured from the time the cancer is diagnosed. Yet treatment in country B is no worse than in country A.