Answer: Use a new "pyramid" sampling method developed by a Cornell University sociologist. The Centers for Disease Control and Prevention (CDC) will use the method to recruit injection drug users (IDUs) and measure their HIV-risk behavior in the 25 U.S. cities with the largest number of AIDS cases.

The sampling method, called respondent-driven sampling (RDS), combines "snowball sampling" (identifying a set of initial respondents, who recruit their peers into the study, and each new set of respondents then recruit their own peers) with a mathematical model that weights the sample to compensate for the fact that it was obtained in a non-random way.

"The statistical method enables researchers to provide both unbiased population estimates and measures of the precision of those estimates," explains Douglas Heckathorn, professor of sociology at Cornell. He developed RDS in 1997 for a National Institute on Drug Abuse HIV-prevention research project targeting drug users in several Connecticut cities. "When applied in a way that fits the mathematical model on which RDS is based, its results have proven to be unbiased for samples of meaningful size," he says.

RDS is already used by the CDC's Global AIDS Program to survey IDUs in Bangkok and IDUs and commercial sex workers in Vietnam. It is also being used by Family Health International, the largest non-profit agency in international public health, in more than a dozen countries and provinces, including Bangladesh, Myanmar (Burma), Cambodia, Egypt, Honduras, India, Kosovo, Mexico, Nepal, Vietnam, Pakistan, Papua New Guinea and Russia to study gay men, IDUs and prostitutes. The National Institute on Drug Abuse also uses RDS to survey IDUs in several cities in
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Contact: Susan S. Lang
SSL4@cornell.edu
607-255-3613
Cornell University News Service
19-Nov-2004