Fault tree analysis is sometimes used to refine hazards to their causes. When software is involved, qualitative fault trees may have some use, but not quantitative ones. Qualitative fault trees depend on the probability of a component failure. With software, there is no way to derive this number. What is the probability that an if statement fails? What does it mean for a branch structure to fail? More realistically, what is the probability of a design error in software? This is not a question with a good answer yet. 10-9 gets thrown around a lot as a figure for software, but no one has offered good evidence explaining on what basis that number was used.