I don't know off-hand, but probably a longitudinal survey of cases and pretty rough estimates. It is probably made a bit easier by there being some big chunks w

I don't know off-hand, but probably a longitudinal survey of cases and pretty rough estimates. It is probably made a bit easier by there being some big chunks which are (likely) more clearly attributable to behavioral choices: smoking-related, sun exposure, and probably some occupational (chemical exposure, etc.).