If you want to have conviction in the answer, you’ll need to think surprisingly carefully about how you approach this question. When you next get a headache, you’ll make a record of how you feel when it first starts. Unfortunately, just examining how you feel after taking aspirin a single time won’t be adequate, since the aspirin may be more helpful some times and less helpful others. That way, you can simply look at all the scores you got just before taking aspirin and average them together. But without such rules, studies are hard to trust, even when they’re just trying to answer a simple question about aspirin.