Frequently this technique is associated with the study of vulnerable or stigmatised population which is reluctant to participate in studies using traditional research methods. Although initial seeds in snowball sampling are in theory randomly chosen, it is difficult to carry out in practice and they are selected via a convenience sampling method. Magnani et al. (2005) pointed that the sample composition is influenced by the choice of initial seeds. Thus, those samples tend to be biased towards more cooperative individuals or those who have a large personal network. According to the limitations observed in the traditional snowball method, the targeted sampling includes an initial ethnographic assessment in order to identify the networks that might exist in a given population. Subgroups are then treated as a cluster sample and reduce the coverage bias and therefore increase the representativeness. In order to improve the external validity of non-probabilistic samples, two methods were developed to approach them to a probabilistic sample. The time space sampling tends to identify accurate subjects in certain types of locations. Researchers enumerate a probability sample of sites, define the time of meeting and the data …show more content…
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