Despite being one of the most common mental disorders, depression is still not well-understood in both research and clinical practice settings. Not all patients present with the same symptoms, which can make it a difficult illness to diagnose. While scientists are hopeful that artificial intelligence can make some order out of the jumble of subjective criteria used to diagnose and treat depression, to date, computational studies still have limitations that have held up the application of machine learning methods in the clinic.This story originally appeared on Massive Science, an editorial partner site that publishes science stories by scientists. Subscribe to their newsletter to get even more science sent straight to you.“Diagnostic heterogeneity,” meaning the broad, non-specific symptoms patients present, has been a long-standing criticism of the American Psychiatric Association’s diagnostic tool, the DSM-V, as well as the various scales used to measure depression severity. For example, the DSM-V allows for a high degree of symptom overlap across multiple disorders, which means that a certain combination of symptoms could be diagnosed as two different disorders, a situation clinicians call comorbidity. However, two individuals could also share the same diagnosis with little — if any — symptom overlap. This raises concerns about the validity of saying they have the same condition, especially since finding the right treatment for an individual with depression is done on a trial-and-error basis that can take months.