Symptom heterogeneity in DSM-IV diagnosis of an episode of major depressive disorder:
DSM-IV major depressive disorder encompasses 9 symptoms;
at least one of the first two symptoms has to be present;
at least 5 symptoms have to be present in total;
3 of the 9 symptoms are bi-directional, e.g. too little vs too much sleep (leading to 12 instead of 9 symptoms);
Conclusion: You can potentially see 1497 patients with the same diagnosis without ever meeting two people with the same symptoms. And that doesn’t take into account things like psychotic depression or melancholic depression, and the fact that depression is often accompanied by anxiety.
Source: Ostergaard, S. D., Jensen, S. O. W., & Bech, P. (2011). The heterogeneity of the depressive syndrome: when numbers get serious.