Diagnosing depression is often handled differently, depending on the study and available data. The most widely accepted method for psychologists is the criteria set forth in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV). This manual uses strict behavioral criteria to diagnose and differentiate between major depressive disorder and other mental disorders such as dysthymic disorder or anxiety disorders. Other studies have used the number of days out of the past 30 that a person has felt hopeless or depressed, to diagnose depression.
There are also a number of ways to summarize depression data. The most common is 12-month prevalence. Below are definitions and examples of prevalence and incidence.
Prevalence: The number of cases with depression at a specified time or a specific span of time. For example, 12-month prevalence of 6% would mean that during a specific 12-month time span, 6% of the population had depression.
Incidence: The number of new diagnosed cases of depression at a specified time or a specific span of time. For example, if the incidence of depression was 2% in 2003, that means 2% of the population were diagnosed with depression who had not been diagnosed previously. The incidence rate measures the probability of developing a disease state during a specified period of time.
Summary of Depression Rates
The table below shows how depression rates have been measured differently over the past 25 years. The past 10 years of research has shown an overall depression rate of about 5-7%.
Definition of depression
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