Grasping Reality on TypePad, by Brad DeLong

Smith and Pell: Parachute use to prevent death and major trauma related to gravitational challenge: systematic review of randomised controlled trials 327 (7429): 1459 -- bmj.com:

No randomised controlled trials of parachute use have been undertaken

The basis for parachute use is purely observational, and its apparent efficacy could potentially be explained by a “healthy cohort” effect"

Source: Justin Wolfers