That helps surveyors make sure that they are getting representative samples.

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In India many more sampling points have been established, and in all countries better survey methods, relying on surveyors knocking on doors rather than asking questions at clinics, have gathered data from more representative samples.

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In face-to-face interviews, teams of researchers surveyed nationally representative samples of people in 18 countries on five continents -- nearly 90, 000 people in all -- and assessed their history of depression using a standardized list of nine criteria.

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Even if the polling firms' samples are representative, they still face a big problem.

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Of course, if the samples are to be representative, interviewers need to contact the same proportion of, say, blue-and white-collar workers as there are in the population as a whole.

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Before turning the samples over to DeCode, a Commission representative coded the tubes to match the codes in the encrypted genealogy database.

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