The Power of Case Studies

In most psychological research, scientists look at large groups to find averages. However, the case study method takes the opposite approach. It is an in-depth, longitudinal investigation of a single individual, group, or event. Instead of looking for a broad trend, researchers act like detectives, gathering every possible detail through interviews, observations, and medical records. In technical terms, we often describe the sample size of a case study as $n=1$. This allows for a level of detail that a survey of 1,000 people could never provide. It is particularly useful for studying rare phenomena that don't happen often enough to form a large test group.

Why don't we just use experiments for everything? The answer is often ethics and rarity. In a controlled experiment, researchers manipulate variables to see an effect. However, you cannot ethically deprive a child of social contact just to see how it affects language development. You also cannot 'create' a rare brain disorder in a lab. Case studies allow psychologists to study these 'natural experiments' that have already occurred. They provide a rich, qualitative description of experiences that would otherwise be impossible to document. This makes them the 'scouts' of psychology, often pointing the way toward new theories that can later be tested on larger groups.

While case studies are incredibly detailed, they have a major weakness: generalization. Because the subject is often unique, we cannot assume that what is true for them is true for everyone else. If one person with a specific brain injury develops a superpower, it doesn't mean everyone with that injury will. Furthermore, because researchers spend so much time with the subject, they may lose their objectivity, leading to researcher bias. They might interpret the subject's behavior in a way that fits their own theory, rather than seeing it clearly. In statistics, we say the results lack 'external validity'—they don't necessarily apply to the wider population $P$.