[epistasis]

When studies are designed a key parameter is the statistical power that the size of the stud provides: how big of a difference in the key stat can it find?

I would expect that the design of a study will be able to find differences in the key readout: did screening reduce death in the types of cancer it could find.

Colon cancer deaths are only a small fraction of total mortality, and statistically, finding a small change in a ratio close to zero is far easier to see than a that same change in a ratio that's further away from zero.

So I would not expect a study to be powered to be able to find changes in total mortality from all causes. That would take a far larger number of people and generally be considered a waste of resources that would be better spent investigating other things.

So I wouldn't take the lack of statistical significance in total mortality as evidence of anything, unless total mortality was a primary outcome that was being tested.

Pretty much all the good interventions for reducing mortality will not have much impact on total mortality because there are so many things causing total mortality. But add up all the small ones and it starts to make differences in total mortality.