| Sample size |
| Differences in imaging methodology |
| Study of interconnected brain systems versus localized regions |
| Diagnostic specificity |
| State versus trait |
| Comorbid substance abuse |
| Cerebral effects of weight loss |
| Treatment status (medications, ECT) |
| Current clinical state |
| Sex |
| Age |
| Episodic number, frequency, duration |
| Illness severity |
| Treatment responsiveness |
| Psychomotor retardation or agitation |
published 2000