SNOWBALL SAMPLING APPLIED TO OPIATE ADDICTS OUTSIDE THE TREATMENT SYSTEM

Sampling opiate addicts outside the treatment system (hidden populatio n) is considered difficult because there is no appropriate sampling fr ame. Several methods to achieve some form of random selection have bee n reviewed by earlier authors. One of these is 'snowballing' or snowba ll sampling. This term, however, covers various methods based on the c entral concept of locating new respondents through introductions from initial contact persons. In this publication theoretical aspects of on e particular type of snowball sampling are described: snowballing with random nominee selection. Experiences with applying this type of snow balling in a medium sized Dutch city will be reported. The characteris tics of the resulting study group of opiate addicts outside the treatm ent system are described together with different kinds of bias that ma y affect the representativeness of the study group.