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