Even when screening is introduced we may still end up selecting inappropriate cases. Nevertheless, these cases may be useful for other purposes. Alternatively, we may decide to discontinue that particular case study since it does not help answer the original research questions.
Cost and access
The number of cases selected, and the particular cases selected, will be affected by the amount of time and money available. Well-executed case studies can be very time consuming. Consequently, only a limited number can be conducted. The cost factor highlights even more the importance of the strategic selection of cases: since only a few case studies can be completed, each one has ‘earn it’skeep’.
Access to cases is the other critical factor that affects both the number of cases studied and the particular cases selected. Certain cases may be identified as being strategically ideal for the design but practical matters may mean that such cases are not available for research purposes. Gaining access to cases that have been identified as strategically appropriate need to be approached with considerable care and effort.
Number of investigators: getting consistency
Case studies may involve a number of different investigators. In ‘Parallel’ designs it is almost inevitable that a number of investigators will be involved-perhaps with a different investigator conducting each study. In multiple case designs each case study should be conducted in a way since inconsistency between cases can invalidate case comparison. Yin (1989) urges the use of case study protocols to help achieve this consistency.