Yes, that would be a retrospective study crunching numbers on roll-overs, which is good information, but to draw a conclusion that seatbelts save 99% of lives is inaccurate. You'd have to repeat all those roll-overs reversing whether the seat belt was worn or not, to draw an accurate conclusion. That's not going to happen. It does in the car industry, they crash test, varying the parameters to gain accurate conclusions.
My statement was more to the point of ... in a situation where there is potential for a rollover, what saves more lives? Seat belts, or something else? I'm saying something else. Straps to prevent a load from shifting to prevent the roll over. Wheel spacers to widen the footprint and increase the amount of force needed to cause a roll over. Properly positioned ballast to prevent the roll over. Going straight up a hill as opposed to ascending at an angle to prevent a roll over. Inspecting the terrain so you can avoid unexpected dips/holes/loose ground/etc so you avoid a roll over. An inclinometer mounted to your dash so you know exactly how far tilted your tractor so you can avoid a roll over. etc, etc.
My point being, a retrospective roll over study does not give an accurate picture of what saves lives in a potential roll over situation. I am more concerned about what is going to happen on my tractor than I am about what has happened in past roll overs and if they were wearing a seat belt or not. I'd rather look at the circumstances that led to the roll over and use safety measures to prevent it than to ignore those factors and wear a seatbelt thinking "the study says I'm 99% safe". I would like to think that most of us here would rather spend a little more time analyzing the situation, and a little more concentration on what we are doing at any given time to prevent a roll over. The reality is that roll overs still happen, not because of seat belt use or non use but because of not analyzing the situation and not concentrating of what is happening.
I hope that the next time anyone is in a situation where there is the potential for enough force to cause a roll over, they stop and think and do what they can to prevent it. Stay safe and make wise choices out there. Wear a seat belt, wear a helmet, install and airbag if you wish, but don't rely on them to save your life. Rely on yourself and your choices.