(2004; 98 pages)
Sampling involves the selection of a number of study units from a defined study population. When drawing a sample, a researcher first needs to decide which population (s)he intends to study. This depends on the research objectives and questions. Sampling strategies need to be defined as you can rarely cover every person in the selected population. In qualitative studies they aim to identify information-rich cases or informants. Information-rich cases are those from which one can learn a great deal about issues of central importance to the purpose of the research, so the term purposeful sampling is used when such people are selected.
For example, when understanding is needed of how infertile women cope, in-depth interviews should be conducted with women who experience infertility. Probability sampling typically depends on large samples selected randomly. A truly random and statistically representative sample allows for generalization from the sample to the larger population. The purpose of such sampling methods is not to gain in-depth understanding of an issue, but to be able to generalize findings. Such sampling can be stratified to ensure that all groups of interest are included.