Katrien Bollen, ‘Depression too often reduced to a checklist of symptoms’, http://www.kuleuven.be/english/news/2015/depression-too-often-reduced-to-a-checklist-of-symptoms, KU Leuven (23 October 2015).

Extremely heterogeneous

We need to stop thinking of depression as a disease that causes a number of interchangeable symptoms. (…) Depression is a complex, extremely heterogeneous system of interacting symptoms. And some of these symptoms may be far more important than others. (…) Clinicians and researchers can learn a lot from paying more attention to individual symptoms and their interactions.