That's a problem if you want to compare the effectiveness of specific treatments. AKA, is doing A, or B, better than C. Or even more basically are screenings useful?
If you want to look overall you can look at the number of people dying at each age of each type of cancer independent of both diagnosis and treatment. AKA how many 43 year old women died of breast cancer. That also has some problems for people that died of cancer before it was detected as cancer, or people who died of suicide or related complications but not necessarily cancer on it's own. Even more critical is reduction in the rate people get cancer in the first place.
Still we are not talking about a small gap, when you start seeing a 30+% drop for a wide range of cancers it's easy to see that yes treatments are extremely useful. Even if you only get an extra say 2 years that's still 2 years to die of a car crash and not cancer.
If you want to look overall you can look at the number of people dying at each age of each type of cancer independent of both diagnosis and treatment. AKA how many 43 year old women died of breast cancer. That also has some problems for people that died of cancer before it was detected as cancer, or people who died of suicide or related complications but not necessarily cancer on it's own. Even more critical is reduction in the rate people get cancer in the first place.
Still we are not talking about a small gap, when you start seeing a 30+% drop for a wide range of cancers it's easy to see that yes treatments are extremely useful. Even if you only get an extra say 2 years that's still 2 years to die of a car crash and not cancer.