 At an effectiveness score ≤ 50, human factors accidents were 65 percent more likely than chance.Below an effectiveness score of 70, accident cause codes indicated the kinds of operator errors consistent with fatigue, confirming that the relationship between accident risk and effectiveness was meaningful.Further analysis indicated that SAFTE/FAST also predicted an increase in accident severity; human factors accidents that occurred when average effectiveness was calculated to be less than 77 were 2½ times more costly than similar accidents that occurred when effectiveness was greater than 90.