IB Psychology SLTopic 5 — Methods of ResearchPaper 1 & 2Core skill~9 min read
Case Studies: Depth Over Breadth
Every other method in this topic trades detail for numbers. A case study does the opposite: one person, or one small group, studied in enormous depth for months or years. It is the only way psychology gets to look closely at something that almost never happens.
📘 What you need to know
A case study is a detailed, in-depth investigation of one individual or a small group.
It is used for the rare and the unusual: unique injuries, unusual conditions, extraordinary experiences.
It uses several methods at once — interviews, observations, tests, records — which is called triangulation.
It produces both qualitative and quantitative data.
Most case studies are longitudinal: the same person is followed over a long period.
Findings cannot be generalised to wider populations, because the sample is one.
Closeness between researcher and participant threatens objectivity.
Why one person can be worth a thousand
Suppose you want to know what part of the brain builds new memories. You cannot randomly assign volunteers to have that part removed. So when a single patient loses it through surgery or injury, that one person becomes the most informative source available anywhere.
The most famous example is the patient known as HM, who had part of both temporal lobes removed to control severe epilepsy and afterwards could no longer form new long-term memories. Researchers worked with him for decades, using interviews, observations, memory tests, IQ tests and later brain scanning. From one man, psychology learned that the hippocampus is essential for turning experiences into lasting memories.
The logic to state in an exam: when something breaks in an unusual way, it shows you what that part was doing while it worked. That is why a single case can tell you about how everyone else functions.
Depth against breadth
This single picture answers most “compare a case study with another method” questions. Depth and generalisability are bought with the same budget.
Triangulation: why case studies use several methods
A case study almost never relies on one source. The researcher interviews the person, observes them, gives them standardised tests, talks to family, and reads records. Where different sources point to the same conclusion, the finding is far harder to dismiss.
Triangulation can also mean more than one researcher analysing the same material, which is the usual way of checking that an interpretation is not just one person’s view.
If a question asks how a case study could be made more credible, triangulation is your first answer. Say what would be triangulated — two data sources, or two researchers — rather than just naming the word.
Longitudinal by nature
Case studies usually run for a long time, sometimes decades. That is not an accident. If you want to know how someone recovers, adapts or changes, you have to be there while it happens. A single visit would only give you a snapshot.
The long timeline brings its own problems: participants drop out or die, the original researchers retire, and methods that were standard at the start look outdated by the end. Findings from an old case study may also transfer poorly to today.
Strengths and limitations
Strengths
Rich, detailed data with real explanatory power.
Holistic: the whole person is considered, not one isolated variable.
High ecological validity — this is a real life, not a task in a room.
Allows study of conditions too rare or unethical to create.
Can overturn a theory: one clear counter-example is enough.
Limitations
Cannot be generalised: the sample is one unusual person.
Cannot be replicated, so reliability cannot be checked.
Researcher closeness threatens objectivity.
Often relies on memory and retrospective accounts.
Expensive and slow in time and money.
🤔 How a single case can still change a whole theory
Generalising from one person is unsafe. But a single case can still disprove a general claim. If a theory says every memory depends on one system, and one patient can still learn new skills while forming no new conscious memories, the theory has to change. That asymmetry — weak for proving, powerful for disproving — is a strong evaluation point that very few students make.
Worked examples
WORKED EXAMPLE
Justify the choice of a case study
A researcher wants to investigate the effects of extreme social isolation in early childhood. Explain why a case study is the most appropriate method here. [4]
Step 1: Rule out the experimentYou could never ethically isolate a child to create the IV.
Step 2: Rule out the survey
Cases are far too rare to gather a sample large enough for a questionnaire.
Step 3: Say what the case study offers
Depth over years, several methods triangulated, and the person’s whole situation considered.
A longitudinal case studyadd the cost: the findings will not generalise, because this child’s circumstances are unique
WORKED EXAMPLE
Evaluate a conclusion drawn from one case
After studying one patient with memory loss for eight years, a research team concludes that “the hippocampus is required for forming new long-term memories in all humans”. Evaluate this conclusion. [4]
Step 1: Give the study its credit
Eight years of triangulated data give a detailed, well-evidenced account of this patient.
Step 2: Attack the word “all”A sample of one cannot support a claim about everyone.Step 3: Note what else may be doing the work
Surgery is rarely limited to one structure, so other damaged areas could contribute.
Step 4: Say what would strengthen it
Converging evidence from other patients and from brain-imaging studies of healthy people.
Strong evidence, over-stated conclusion“evaluate” wants both sides — never write only the criticism
💡 Exam tip
Use the word triangulation and explain it. It is a high-value term that students often skip.
Say “cannot be generalised” and then say why: the participant was chosen precisely because they are unusual.
Mention that case studies produce both qualitative and quantitative data. Many students wrongly say qualitative only.
Longitudinal is a strength and a weakness at once. Say both in the same sentence to show balance.
If asked to compare with an experiment, use the depth-versus-control framing rather than listing facts.
Name the ethical duties: informed consent from the person or a guardian, confidentiality, and the right to withdraw even years in.
⚠️ Common mix-up
Thinking a case study is one method. It is a design that uses several methods together.
Case study vs case example. Mentioning a study in an essay is not conducting a case study.
Saying case studies have no quantitative data. Test scores and scan measurements are quantitative.
Confusing generalisability with usefulness. A finding that does not generalise can still be extremely important.
Assuming small sample is the only limitation. Objectivity and replication are the stronger criticisms.
Forgetting the participant is a person. Long-term studies of vulnerable individuals raise real, ongoing consent issues.
Up next: Reliability: Getting the Same Result Twice — the first of the design ideas you use to judge every method in this topic.
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