Nearly one American in 10 suffers from depression, about a third of whom suffer from a severe form of the disease, a study published Thursday found.

Nine percent of more than 235,000 adults polled in 45 US states, the capital capital Washington, Puerto Rico and the US Virgin Islands, met the criteria for depression, according to researcher with the US Centers for Disease Control and Prevention (CDC).

The CDC said that its survey, conducted from 2006 and 2008 found that 3.4 percent of test subjects suffered from what it described as "major depression."

The main factor affecting depression was a person's employment status, the researchers found. For example, the study found that only two percent of people with jobs suffered from major depression.

The second biggest contributing factor to depression was education: 6.7 percent of people with less than a high school diploma suffered major depression, compared to four percent of high school graduates and just 2.5 percent of people who had spent at least some time at university.

Whether a person was depressed or very depressed was determined by their responses to a questionnaire, which asked participants how often during the previous two weeks they had experienced feelings of hopelessness or disinterest, had trouble falling asleep or were sleeping too much.

Other questions asked if their appetite was poor or, on the contrary, if they were overeating; if they felt tired all the time; felt bad about themselves or felt a failure; had trouble concentrating; and suffered from restlessness or lethargy.

Participants who met five of the eight criteria, including having little interest or pleasure in doing things or feeling down or hopeless, for more than half the days of the preceding two weeks, were deemed to have major depression.

If they met up to four of the eight criteria, including lack of interest or feelings of hopelessness, they were deemed to have "other", less severe depression.

An earlier study, conducted between 2001-2002, found that 6.6 percent of the US adult population had experienced major depression during the previous 12 months, the CDC said in its weekly Morbidity and Mortality report.