NASA has its own specifications for man-rated equipment, that is equipment that's rated to carry human beings into space. IIRC the standard is "5 nines" (99.999%), or one failure in 100,000 as an acceptable failure rate.

It's not like we don't posess the technology to make spaceflight any safer--we do. The only thing that's causing these failures that result in loss of human life has been people who decided to cut corners. The Apollo 1 capsule had faulty wiring that the vendor failed to repair; the Apollo 13 oxygen tank heater was damaged, and the vendor failed to replace it; the Challenger solid booster had temperature limitations that the vendor failed to alert the launch team of. IMHO there's no excuse for putting human life unnecessarily at risk, especially when it's to save a buck.