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From: Gary Casey <glcasey@adelphia.net>
Subject: Safety statistics

I think it's been laid to rest that commercial aviation is safer that  
general aviation, but there seems to be an argument as whether flight  
hours or passenger miles is the best yardstick.  I think it doesn't  
matter as long as the aircraft population in the group is reasonably  
homogeneous.  Let's fly 4 airplanes, 2 freighters and 2 passenger  
planes and the same route.  One of each crashes.  Are freighters more  
dangerous on either of the two methods of measurement?  The math  
comes out the same - the successful passenger plane maybe got 500  
people safely to the destination, but the one that crashed cut the  
success rate in half.  It doesn't matter whether you combine the  
statistics for freighters and passenger aircraft or not.  But what  
happens if the population isn't so similar.  Say freighters crash a  
lot more often - both freighters crashed and both passenger planes  
got there safely.  Now combining the statistics on a flight hour  
basis makes the passenger service look really bad, but combining them  
on a passenger mile basis only makes passenger service look a little  
worse.  The 15:1 ratio that Brent talked about would look even worse  
if you converted it from flight hours to passenger miles.  No  
question, we have met the enemy and he is us.

Gary Casey