Two different psychiatrists assessed each patient, and the agreement between their diagnoses was calculated, as the kappa statistic, where 0 indicates no correlation at all and 1 is perfect.
It turns out that the reliabilities of most DSM-5 disorders were not very good. The majority were around 0.5, which is at best mediocre. These included such pillars of psychiatric diagnosis like schizophrenia, bipolar disorder, and alcoholism.
Others were worse. Depression, had a frankly crap kappa of 0.28, and the new ‘Mixed Anxiety-Depressive Disorder’ came in at -0.004 (sic). It was completely meaningless.