insight can be gained from decoding the symptoms in detail.
Drilling down into the world of Depression there is a common clinical test called the Beck Depression Inventory (BDI). This is a test of 21 questions that allows a doctor to score how depressed an individual feels. This is important as the doctor gains quick insight into the current status of the unhappiness of a patient. Naturally, it is not 100% accurate. Clinical testing shows it is about 90% accurate, because it is impossible to turn human behavior into pulleys and software.
For example, after my injury, I was frightened of having people see me as different. Even though I was brain injured, I was smart enough to pick the best answer in order to make me seem healthy. In reality, I was too scared and anxious to be honest. I was one of those 10% of people who threw off the Beck Depression Inventory.
So, the BDI measures the depth and intensity of depression in patients with psychiatric diagnoses.
The BDI has extensive clinical value, and has measured the "how much" a person is depressed since its original creation in 1961 by Aaron Beck.