Diagnosis of mental illness is done by interview. There is no objective lab test demonstrating a "biochemical imbalance" in the brain. Diagnosis is subjective.
Compared to the ATOS (the independent assessors) who are real doctors but who are forced to operate to a very strict, limited, set of tick boxes. That kind of works for some physical disorders.
"On my bad days I cannot stand for more than 30 minutes"; "On my bad days I cannot walk unaided more than 10 metres".
Thus, real doctors are not allowed to use their subjective experienced judgement.
http://en.wikipedia.org/wiki/Atos#Controversy_over_disabilit...