Brad Delong points us to a study published in the British Medical Association jounal BMJ and quotes from it:
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”
The full journal article is well worth following at the link he quotes. Besides the laugh (warning: the positive health effects of laughing haven’t been proved by randomized controlled trials either), the authors suggest by parody an excellent point. Sometimes rigid adherence to one single methodology in science is sometimes not only uncalled for and useless, it can also be immoral. Some Austrian and New Classical economists might want to take note.