Yes, and do you know how much variability exists between practitioners? Diagnosis is largely a matter of personal opinion, not science.
The reason for that, in turn, is because no on knows what causes depression -- it's not like a medical condition with an identified pathogen and clinically validated treatment.
But the vague state of depression as an illness with an unknown cause doesn't prevent psychiatry and clinical psychology from offering therapy and drugs of dubious efficacy.
Antidepression drugs have been shown to be ineffective in most cases --
Link: http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fj...
Title: "Initial Severity and Antidepressant Benefits: A Meta-Analysis of Data Submitted to the Food and Drug Administration"
Quote: "Meta-analyses of antidepressant medications have reported only modest benefits over placebo treatment, and when unpublished trial data are included, the benefit falls below accepted criteria for clinical significance."
The story is that drug companies publish only studies that support their belief in their drugs' efficacy. But when completed but unpublished studies are included, the evidence for the efficacy of antidepression drugs evaporates.
The study's authors don't say the drug companies deliberately suppressed negative findings -- they leave that conclusion to the reader.