There is also no good study showing that parachutes prevent death when falling from an airplane. The best we have is observational "evidence" where there are many uncontrolled variables, such as people whose parachutes failed (but they may be unlucky or careless in other ways, or incompetent fallers). And then speculative physics models involving rates of acceleration and energy transfer, which can hardly be assumed to carry over to the real world in the absence of a proper large-scale randomized test.