- History and physical exam are extremely subjective.
- Medical notes and records can be massaged to sound better than reality (The patient didn't have a stroke after their operation. They had "minor lower limb weakness which we anticipate will improve over the next few weeks") or attributed to all sorts of things (patient's cognitive function hasn't worsened, they have some "fatigue" following surgery).
- People can be very selective about which studies are ordered.
- Junior and ancillary staff are reluctant to report poor outcomes about established senior people.
I'll give a real-life example: Professor X is a "world-renowned expert" on neurosurgery at an academic mecca. He claims a "1-2%" complication rate on aneurysm surgeries in the clinic and at any conferences. His residents know he performs outdated surgeries with spectacularly bad outcomes but no one would be willing to talk about this openly unless they want to torpedo their career prospects.