Generalizing from a single case
Saturday 8 September 2012
Misconceptions in psychology
One cannot generalize on the basis of an individual case study.
More discoveries have arisen from intense observation [of individual cases] than from statistics applied to large groups.
William Beveridge (1951)
The view that one cannot generalize on the basis of a single case is a common misunderstanding seen on examinations. It is often used as an evaluative strategy. For example, “HM was a case study. One limitation is that case studies cannot be generalized.” This is actually not true.
It is true that cases are not randomly selected. But then, they are not intended to be random. Purposive sampling is used for case studies. Case study approaches do not produce statistically representative data in this manner. The question is, does this matter?
Formal generalization is only one of many ways by which people gain and accumulate knowledge. That knowledge cannot be formally generalized does not mean that it cannot enter into the collective process of knowledge accumulation in a given field or in a society. Knowledge may be transferable even where it is not formally generalizable. A purely descriptive, phenomenological case study has often helped cut a path toward scientific innovation.
The case study is ideal for generalizing using the type of test that Karl Popper called “falsification.” If just one observation does not fit with the theory, it is considered not valid and must therefore be either revised or rejected. Deviant cases and the falsifications they entail are main sources of theory development, because they point to the development of new concepts, variables, and causal mechanisms.
A single case study that is supported by other case studies is usually considered more dependable. When another case reaches the same conclusions, it confirms the findings of the first case. The question of transferability is perhaps more relevant than the question of generalizability. Does a case study of one school transfer to another school? The answer is that it depends on how similar the two schools are.
So, back to HM. Students could say that one of the problems with HM is that although he sustained damage to the hippocampal regions, it is difficult to transfer the results of the case for at least two reasons: First, prior to the surgery he was suffering from epileptic seizures which could have theoretically caused memory loss; Second, HM was put on medications which could have resulted in memory loss. However, further case studies of individuals with hippocampal cell loss or damage have supported the findings, so in fact this study may be “generalized.”