A case study focuses on a particular person or group, and therefore can provide rich and detailed qualitative information. It's not a method itself, as researchers use other methods to acquire a wide range of information that all collectively contribute to a case study. For instance, observing a person and interviewing them could be used in conjunction with each other to produce detailed information on the case in question.
There is an ethical requirement for the person/s conducting a case study to be competent and professional, because they very often disect the lives of individuals to such an extent that their privacy and wellbeing could be compromised if the information isn't handled appropriately.
The key strengths are:
- It can provide rich qualitative information.
- Allows a researcher to study situations that would otherwise be impractical or unethical to bring about artificially.
- They have maximum ecological validity because they occured naturally, and the researcher is studying this occurence retrospectively.
The key weaknesses are:
- Because it's so specific to a particular case, the results cannot be generalised to the population as a whole.
- Because the case is so unique, it's impossible to replicate.
- Due to the detail required it's often very time consuming to conduct a case study.
You could mention other factors which influence case studies. For example, many case studies involve observation or interviews, and the weaknesses of these individual methods could be explored; researcher bias, for instance.
Research examples of case studies:
Curtiss (1977) - Case study of Genie, a child who was kept in almost total isolation until she was discovered aged 13. Clearly it would have been unethical to put her in this situation intentionally, but studying such can give a good insight into the effects of long-term isolation and maternal privation. For instance, Genie had no language skills, wasn't toilet trained, couldn't walk properly and had the social maturity of the average two-year-old. She showed some signs of improvement after she was discovered, but never became anything resembling normality.
Freud (1909) - Little Hans case study of a boy who had a phobia of horses. After careful consideration of all the evidence and - using his psychoanalytic approach to atypical behaviour - Freud concluded that the boy's real problem was an unconcious fear of his father.
There are lots of others, but I'd recommend the above two because of the range of Psychology topics they cover in the Higher.
Hope it's helpful.