Besides, if someone tells you they're in pain, THEY're the one doing the introspection, not you. What you're inspecting is their behaviour. Seeking insight into people's thoughts by asking them questions is interrogation, not introspection.
People can only introspect their own thoughts.
If introspection really allowed us to study experience, the way geologists study rocks and botanists study plants, then you ought to be able to replicate my observations about my experience. But you can only make observations about your own experience, not mine. There's no problem with people reporting different experiences and many people reporting similar ones.