A detailed study of an individual or small group of people.
Case studies are particularly useful in revealing the origins of abnormal behaviour. In fact some forms of psychotherapy rely on building up a long and detailed case history as an aid to understanding and helping the client. Case studies allow psychologists to look at people in situations which we could not possibly have engineered e.g. recovery from illness. Case studies usually provide an in-depth picture producing rich data and sometimes produce quantitative data too. A major strength of case studies is that they often relate to a participants real life.
However case studies only relate to one individual (or a small group of people) and we therefore have to be careful generalising from the results. It is difficult knowing how typical the individual is. If the study is retrospective (if the individual is asked to look back over his/her life) then memory may not be accurate and indeed, people may deliberately mislead the researcher. The data may therefore be unreliable. It is difficult to control variables and the close relationship between researcher and participant may introduce bias. We can not usually make cause and effect statements and of course case studies are time consuming and therefore expensive.