Definition	Depressive disorders are characterized by persistent low mood, loss of interest and enjoyment, and reduced energy. They often impair function. Older adults: Older adults are generally defined as people 65 years or older. The presentation of depression in older adults may be atypical; low mood may be masked and anxiety or memory impairment may be the principal presenting symptoms. Dementia should be considered in the differential diagnosis of depression in older adults.1
Incidence/Prevalence	Younger adults: Depressive disorders are common, with a prevalence of major depression between 5 and 10 percent of people seen in primary care settings.2 Two to three times as many people may have depressive symptoms but do not meet criteria for major depression. Women are affected twice as often as men. Depressive disorders are the fourth most important cause of disability worldwide, and they are expected to become the second most important cause by the year 2020.3,4 Older adults: Between 10 and 15 percent of older people have significant depressive symptomatology, although major depression is relatively rare in older adults.5
Etiology/Risk Factors	The causes are uncertain but include childhood events and current psychosocial adversity.
Prognosis	About one half of people suffering a first episode of major depressive disorder experience further symptoms in the next 10 years.6 Different levels of severity7,8 indicate different prognosis and treatment. Mild to moderate depression is characterized by depressive symptoms and some functional impairment. Many people recover in the short term, but about one half experience recurrent symptoms. Severe depression is characterized by additional agitation or psychomotor retardation with marked somatic symptoms. In this review, treatments are considered to have been evaluated in severe depression if the randomized control trials (RCTs) included inpatients. Psychotic depression is characterized by additional hallucinations, delusions, or both. Older adults: The prognosis may be especially poor in elderly people with a chronic or relapsing course.9
Clinical Aims	To improve mood, social and occupational functioning, and quality of life; to reduce morbidity and mortality; to prevent recurrence of depressive disorder; and to minimize adverse effects of treatment.
Clinical Outcomes	Depressive symptoms rated by the depressed person and clinician, social functioning, occupational functioning, quality of life, admission to hospital, rates of self harm, relapse of depressive symptoms, rates of adverse events. Trials often use continuous scales to measure depressive symptoms (such as the Hamilton Depression Rating Scale and the Beck Depression Inventory). Clinician reports and self-reported global outcome measures are also used. Changes in continuous measures can be dealt with in two ways. They can be dichotomized in an arbitrary but clinically helpful manner (e.g., taking a reduction in depressive symptoms of more than 50 percent as an end point), which allows results to be expressed as relative risks and numbers needed to treat. Alternatively, they can be treated as continuous variables, as is done for systematic analysis. In this case, the pooled estimate of effect (the effect size) expresses the degree of overlap between the range of scores in the control and experimental groups. The effect size can be used to estimate the proportion of people in the control group who had a poorer outcome than the average person in the experimental group. A proportion of 50 percent indicates that the treatment has no effect. Older adults: The Hamilton Depression Rating Scale is not ideal for older people because it includes a number of somatic items that may be positive in older people who are not depressed. It has been the most widely used scale, although specific scales for elderly people (such as the Geriatric Depression Scale) avoid somatic items.