The Patient Health Questionnaire-9 (PHQ-9) is a quick, 10-question test that attempts to objectify an inherently subjective topic: depression. You can take the test here.
Each question asks about your mental health and various behaviors over the past two weeks. Based on your response, you receive a score of 0-3 for each question. At the end of the test, the responses are tallied and a "depression score" is assigned as follows:
0-4: None or minimal
15-19: Moderately severe
The test is far from comprehensive. It does not, for instance, ask about anger, anxiety, or irritability, all of which can be symptoms of depression. Thus, the test should be thought of as a quick-and-dirty screen rather than an in-depth assessment.
Using this test, the CDC has been tracking depression for several years. A new report reveals its prevalence among American adults aged 20 and over:
As shown, 76.3% had no or minimal depression, while a substantial proportion of Americans suffered mild (15.6%), moderate (5.1%), or severe (2.9%) depression. Combined, 8% (roughly 1 in 12) Americans have moderate to severe depression. And, as is typical, women are more likely to be depressed than men.
However, given that the CDC used the PHQ-9 as its measuring tool, it is likely underestimating the true extent of the problem. In 2013, the Los Angeles Times wrote:
A new study has found that people suffering a major depressive episode who report they have become grouchy, hostile, grumpy, argumentative, foul-tempered or angry will likely have a "more complex, chronic and severe form" of major depressive disorder than those who do not acknowledge irritable feelings and behavior.
The point is that feeling sad or worthless is not the only way in which depression manifests itself. Being perpetually angry at the world is another face of depression. If social media is any indication, there may be a lot more depressed people out there than the CDC realizes.
Source: QuickStats: Percentage of Adults Aged ≥20 Years Reporting Depressive Symptoms in the Past 2 Weeks, by Sex — National Health and Nutrition Examination Survey, United States, 2013–2016. MMWR 67 (9): 287. Published 9-March-2018. DOI: 10.15585/mmwr.mm6709a5.