It's a good question, which has been extensively studied over the last few decades in the medical context. The industry phrase would be "Patient Reported Outcom

It's a good question, which has been extensively studied over the last few decades in the medical context. The industry phrase would be "Patient Reported Outcomes" if you'd like to explore further. After many papers and lots of clinical trials, it's widely accepted in the scientific community that the answers to both of your questions are "yes". E.g. see this FDA paper[1]. Of course, it's possible to design your questionnaire badly and there's an art to this, so it's right to be skeptical/cautious about putting this into practice. But you seem to be getting at a more fundamental epistemic question about whether the approach is possible even in principle, which has been thoroughly investigated. [1]: https://www.fda.gov/media/77832/download https://www.fda.gov/media/77832/download