Determining how many people have a mental illness can be difficult, and prevalence estimates vary. While numerous surveys include questions related to mental illness, few provide prevalence estimates of diagnosable mental illness (e.g., major depressive disorder as opposed to feeling depressed, or generalized anxiety disorder as opposed to feeling anxious), and fewer still provide national prevalence estimates of diagnosable mental illness. This report briefly describes the methodology and results of three large surveys (funded in whole or in part by the U.S. Department of Health and Human Services) that provide national prevalence estimates of diagnosable mental illness: the National Comorbidity Survey Replication (NCS-R), the National Comorbidity Survey Replication Adolescent Supplement (NCS-A), and the National Survey on Drug Use and Health (NSDUH). The NCS-R and the NCS-A have the advantage of identifying specific mental illnesses, but they are more than a decade old. The NSDUH does not identify specific mental illnesses, but it has the advantage of being conducted annually.

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