quacks
if a doctor were a quack, how would they know? suppose i don't like this doctor's diagnosis and/or cure, i go to a different doctor. and i never see the first doctor again. it would be really easy for the first doctor to assume their treatment helped that missing patient. doctors never get negative feedback. only positive feedback. which would skew their world view away from reality. which might explain some of the hubris i've observed in some of the medical profession.