A National Depression Index for Australia.

OBJECTIVE To develop a National Depression Index for measuring the depression status of the Australian population. DESIGN Cross-sectional data were analysed from two random samples of the Australian adult population - the National Survey of Mental Health and Wellbeing (2000) and the National Health Survey (2001). PARTICIPANTS The National Survey of Mental Health and Wellbeing (2000) - 10 641 participants; and the National Health Survey (2001) - 17 918 participants. MAIN OUTCOME MEASURES Selected items from the Kessler Psychological Distress Scale (K10); and diagnoses of a major depressive episode according to DSM-IV criteria using a computerised interview. RESULTS Six items from the K10 that were most closely related to the DSM-IV diagnosis of "major depressive episode" were identified. Scores on an index calculated from these items were rescaled to form an index reflecting relative risk of depression and having a value of 100 for the Australian adult population. Taking into account sex, employment status and income, index values were higher in younger people, females, unemployed people and those socioeconomically disadvantaged. This pattern provides additional support for the validity of the index, as well as establishing benchmark levels to which index values from future surveys and in other groups may be compared. CONCLUSIONS The proposed National Depression Index is a valid indicator of depression and level of depressive symptoms. It is suitable for monitoring depression at the population level. The scaling characteristics of the measure ensure that it can be interpreted by members of the general public.