Yin (2005) suggested that researchers should decide whether to do single-case or multiple-case studies and chose to keep the case holistic or have embedded sub-cases. This two-by-two combination can produce four basic designs for case studies.
When selecting a case for a case study, researchers often use information-oriented sampling , as opposed to random sampling [3]. This is because an average case is often not the richest in information. Extreme or atypical cases reveal more information because they activate more basic mechanisms and more actors in the situation studied. In addition, from both an understanding-oriented and an action-oriented perspective, it is often more important to clarify the deeper causes behind a given problem and its consequences than to describe the symptoms of the problem and how frequently they occur. Random samples emphasizing representativeness will seldom be able to produce this kind of insight; it is more appropriate to select some few cases chosen for their validity. But this isnt always the case.
Three types of information-oriented cases may be distinguished:
Extreme or deviant cases
Critical cases
Paradigmatic cases.
[edit]Critical case
A critical case can be defined as having strategic importance in relation to the general problem. For example, an occupational medicine clinic wanted to investigate whether people working with organic solvents suffered brain damage. Instead of choosing a representative sample among all those enterprises in the clinic’s area that used organic solvents, the clinic strategically located , ‘If it is valid for this case, it is valid for all (or many) cases.’ In its negative form, the generalization would be, ‘If it is not valid for this case, then it is not valid for any (or only few) cases.’

For more on case selection, see [1]
[edit]Generalizing from case studies

The case study is