It takes a lot of training to learn how to apply these simple criteria perfectly. During their training, psychiatrists spend hours every day for months on end at the hospital, interviewing one volunteer patient after another in the company of a panel of experienced clinicians. After the student doctor makes and explains his or her diagnosis, each of the experienced doctors discusses why that diagnosis was right or wrong. This continues for years until doctors are proficient in the complex art of diagnosis.
It is essential that everyone associated with unipolar major depression know and understand the official diagnostic criteria. Just knowing how unipolar major depression is defined will greatly help you to understand it. However, a definitive diagnosis requires a one-on-one examination by a trained doctor, not just self evaluation.