This produces a statistical association but, especially with genomic factors, factor y can just be something that randomly identifies the group.
Similarly in this data set you might find a group of people who have higher strength because of genomic factors that make it easier for them to build strength from the same workouts as "normall" people. These people might also come from a subpopulation (farm famlies, people of Slavic heritage who knows) that has lower rates of cancer for genomic or cultural reasons. Even a small number of such people can make the association look significant though it will disappear once you remove them.
It could also be something like a certain food or diet making it both easier to build strength and preventing cancer.
Some of these results are probably valid but if you want to scientifically extend your life you need to do so with statistical rigor.
I don't believe you make every, or even remotely close to most of your decisions in daily life by this standard. Human brains aren't designed to and don't have the energy to do so.
However, in my job, I analyze large medical datasets. You will understand if I get a bit pedantic about such things.
Doing some strength training is a probable plus on several fronts with few risks. However if I am advising doctors or the presidential physical fitness council to recommend strength training vs endurance training vs focusing on diet on an institutional level I need to look at the evidence for each with more rigor.
So endurance running, which I would guess you would categorise as marathon+ will decrease your life expectancy.
Edit: /s/will/could Depending on how much running you already do.
Exercise for higher quality of life and a better looking body, the longevity benefits are overrated.
As Edward Tufte says, "Correlation is not causation but it sure is a hint." That increased strength improves life expectancy should be the null hypothesis. At least at the one end, extreme frailty is a common eye test for poor health. Since this isn't a scientific journal here, but a discussion, I think most of us would agree that stronger people often have more resources to ward off disease, cope with temporary reductions in nutrient bioavailability longer, can defend themselves from threats more easily, and can avoid more dangerous falls by maintaining their center of balance. And that's not a comprehensive list.
Personally, I'm satisfied that increased strength reduces all cause mortality and I believe the link is strong enough to be actionable now. I'm an evangelist for it among my older friends and relatives. One way strength reduces deaths is by preventing common injuries, e.g., falling and breaking a hip. A recent study showed that a simple graded test of sitting on the ground to standing could differentiate mortality risk sharply. While we can't measure how much strength and how many years it adds, not least because that's just a vague question, the link here was almost an order of magnitude from the lowest grade to the highest grade for one repetition of standing from the ground.
http://www.cbsnews.com/8301-204_162-57559095/sitting-and-sta...
http://cpr.sagepub.com/content/early/2012/12/10/204748731247...