What a case study is

1. A case study is an in-depth investigation of a single person, group or event.
2. It builds up one case in great detail, drawing on several sources of information about the same person.
3. It combines several methods at once, such as interviews, tests, tasks and observation.
4. Many case studies run over months or years, so change over time can be tracked.
5. The two features that define one are its use of qualitative data and its use of small samples.

Use of qualitative data

1. Qualitative data is data in the form of words, such as what someone says, writes or is seen doing.
2. In a case study it comes from interviews with the person, notes from family and carers, diaries, letters and medical records.
3. Strength: words keep the meaning behind a behaviour, so the account describes the whole person, which raises validity.
4. Weakness: words have to be interpreted, and two researchers can read the same interview differently, so reliability is low.
5. Weakness: words cannot be totalled, so a case study cannot be summarised with a mean or a bar chart.
6. Tests inside a case study add some quantitative data, such as a memory score out of 20 or a brain scan measurement.

Use of small samples

1. The sample is usually one person, or one small group such as a family or a single class.
2. A small sample is often the only option, because rare conditions and one-off events cannot be produced on demand.
3. The cost is population validity, because findings from one unusual person may hold for nobody else.
4. A case study shows that something is possible, but it does not show how common that thing is.
5. A single case can still count against a theory, because one clear exception shows the theory is not the whole story.

Strengths and weaknesses of case studies

1. Strength: it reaches rare cases, such as a specific pattern of brain damage, that could never be created ethically in a laboratory.
2. Strength: several sources allow findings to be cross-checked, so a point supported by the diary, the tests and the family is better evidenced.
3. Strength: the person is studied in their own life rather than in a set task, so ecological validity is high.
4. Weakness: a unique case cannot be repeated, so there is no way to test whether the findings would come out the same again.
5. Weakness: working closely with one person for a long time invites researcher bias, where the detail that fits the theory is the detail noticed.
6. Weakness: much of the data looks backwards, relying on memories of childhood or of an accident, and those memories may be wrong.
7. Weakness: one identifiable person is hard to keep anonymous, so confidentiality takes particular care.

When a case study fits the research objective

1. It fits an objective about understanding a rare or unusual case in depth.
2. It fits an objective about why and how something happened to one person.
3. It cannot produce results that apply to a whole population, because the sample is one case.
4. It cannot produce a figure to compare between two groups, because there is only one group.
5. The findings are reported as coming from one case, with the limits of that stated alongside them.

Turning this into marks

1. When a strength and a weakness are both asked for, keep them as two separate points rather than one sentence balancing depth against generalisability.
2. Point at the feature that causes the weakness, such as a sample of one person, because "it was only a case study" does not secure the knowledge mark.
3. Name the case under discussion, such as Clive Wearing or the Wolfman, and tie the point to what was actually collected from them.
4. The mark most often lost is writing that a case study proves something about everyone, when it shows what is possible in one case.