As part of the WHO World Mental Health Survey Initiative (WMHSI), ten OECD countries conducted large-scale epidemiological surveys between 2002 and 2005. These countries used Composite International Diagnostic Instrument (CIDI) to measure the occurrence of various types of disorders, their nature and intensity, and the treatment provided.
Disorders considered in the surveys include anxiety disorders, mood disorders, disorders linked to impulse control, and disorders due to use of alcohol and drugs. All disorders are classified as serious, moderate, or mild.
The WMHSI data cover all people aged 18 and over. However, the age limit is 16 years in New Zealand, 20 years in Japan and 18-65 years in Mexico. Sample sizes range between 2,000 and 13,000. Response rates vary between 50% and 80%.