Single Case Study Design Psychology Careers

Case Study Method

Saul McLeod published 2008

Case studies are in-depth investigations of a single person, group, event or community. Typically, data are gathered from a variety of sources and by using several different methods (e.g. observations & interviews). The research may also continue for an extended period of time, so processes and developments can be studied as they happen.

The case study research method originated in clinical medicine (the case history, i.e. the patient’s personal history).

The case study method often involves simply observing what happens to, or reconstructing ‘the case history’ of a single participant or group of individuals (such as a school class or a specific social group), i.e. the idiographic approach. Case studies allow a researcher to investigate a topic in far more detail than might be possible if they were trying to deal with a large number of research participants (nomothetic approach) with the aim of ‘averaging’.

The case study is not itself a research method, but researchers select methods of data collection and analysis that will generate material suitable for case studies. Amongst the sources of data the psychologist is likely to turn to when carrying out a case study are observations of a person’s daily routine, unstructured interviews with the participant herself (and with people who know her), diaries, personal notes (e.g. letters, photographs, notes) or official document (e.g. case notes, clinical notes, appraisal reports). Most of this information is likely to be qualitative (i.e. verbal description rather than measurement) but the psychologist might collect numerical data as well.

The data collected can be analyzed using different theories (e.g. grounded theory, interpretative phenomenological analysis, text interpretation, e.g. thematic coding) etc. All the approaches mentioned here use preconceived categories in the analysis and they are ideographic in their approach, i.e. they focus on the individual case without reference to a comparison group.

Case studies are widely used in psychology and amongst the best known were the ones carried out by Sigmund Freud. He conducted very detailed investigations into the private lives of his patients in an attempt to both understand and help them overcome their illnesses. 

Freud's most famous case studies include Little Hans (1909a) and The Rat Man (1909b). Even today case histories are one of the main methods of investigation in abnormal psychology and psychiatry. For students of these disciplines they can give a vivid insight into what those who suffer from mental illness often have to endure.

Case studies are often conducted in clinical medicine and involve collecting and reporting descriptive information about a particular person or specific environment, such as a school. In psychology, case studies are often confined to the study of a particular individual. The information is mainly biographical and relates to events in the individual's past (i.e. retrospective), as well as to significant events which are currently occurring in his or her everyday life.

In order to produce a fairly detailed and comprehensive profile of the person, the psychologist may use various types of accessible data, such as medical records, employer's reports, school reports or psychological test results. The interview is also an extremely effective procedure for obtaining information about an individual, and it may be used to collect comments from the person's friends, parents, employer, work mates and others who have a good knowledge of the person, as well as to obtain facts from the person him or herself.

This makes it clear that the case study is a method that should only be used by a psychologist, therapist or psychiatrist, i.e. someone with a professional qualification. There is an ethical issue of competence. Only someone qualified to diagnose and treat a person can conduct a formal case study relating to atypical (i.e. abnormal) behavior or atypical development.

The procedure used in a case study means that the researcher provides a description of the behavior. This comes from interviews and other sources, such as observation. The client also reports detail of events from his or her point of view. The researcher then writes up the information from both sources above as the case study, and interprets the information.

Interpreting the information means the researcher decides what to include or leave out. A good case study should always make clear which information is factual description and which is an inference or the opinion of the researcher.

Strengths of Case Studies

  • Provides detailed (rich qualitative) information.
  • Provides insight for further research.
  • Permitting investigation of otherwise impractical (or unethical) situations.

Because of their in-depth, multi-sided approach case studies often shed light on aspects of human thinking and behavior that would be unethical or impractical to study in other ways. Research which only looks into the measurable aspects of human behavior is not likely to give us insights into the subjective dimension to experience which is so important to psychoanalytic and humanistic psychologists.

Case studies are often used in exploratory research. They can help us generate new ideas (that might be tested by other methods). They are an important way of illustrating theories and can help show how different aspects of a person's life are related to each other. The method is therefore important for psychologists who adopt a holistic point of view (i.e. humanistic psychologists).

Limitations of Case Studies

  • Can’t generalize the results to the wider population.
  • Researchers' own subjective feeling may influence the case study (researcher bias).
  • Difficult to replicate.
  • Time consuming.

Because a case study deals with only one person/event/group we can never be sure whether the conclusions drawn from this particular case apply elsewhere. The results of the study are not generalizable because we can never know whether the case we have investigated is representative of the wider body of "similar" instances

Because they are based on the analysis of qualitative (i.e. descriptive) data a lot depends on the interpretation the psychologist places on the information she has acquired. This means that there is a lot of scope for observer bias and it could be that the subjective opinions of the psychologist intrude in the assessment of what the data means.

For example, Freud has been criticized for producing case studies in which the information was sometimes distorted to fit the particular theories about behavior (e.g. Little Hans). This is also true of Money’s interpretation of the Bruce/Brenda case study (Diamond, 1997) when he ignored evidence that went against his theory.


Diamond, M., & Sigmundson, K. (1997). Sex Reassignment at Birth: Long-term Review and Clinical Implications. Archives of Pediatrics & Adolescent Medicine, 151(3), 298-304

Freud, S. (1909a). Analysis of a phobia of a five year old boy. In The Pelican Freud Library (1977), Vol 8, Case Histories 1, pages 169-306

Freud, S. (1909b). Bemerkungen über einen Fall von Zwangsneurose (Der "Rattenmann"). Jb. psychoanal. psychopathol. Forsch., I, p. 357-421; GW, VII, p. 379-463; Notes upon a case of obsessional neurosis, SE, 10: 151-318.

How to reference this article:

McLeod, S. A. (2008). Case study method. Retrieved from


A brief overview highlighting key elements of single case design is presented. Four types of single case design are identified. Central elements and the value of the use of single case designs are underscored.

Key words: AB design, ABA design, AB1B2 design, single subject design, changing criterion design, time series design


A wide variety of valid, useful designs exist for the measurement of a single case. These designs are either classified as qualitative or quasi experimental designs as they do not contain the provisions of random, representative samples or randomization to treatment/intervention. This classification convention also occurs because traditional statistical analyses based classical theorems are not used. Regardless of these circumstances, the single case designs provide the practitioner and field researcher with a powerful means of behavioral measurement.1-3

This paper will present elements in an overview of commonly used single case designs: Baseline-Treatment-Return to Baseline (ABA or OXO); Baseline-Treatment (A-B or OX); Baseline-Treatment-Intensified Treatment (AB1B2 or changing criterion); time series (O1O2O3O4 X O5O6O7O8).4 Each of these commonly used designs highlights some of the central features of single case design such as ease of use, immediate feedback to client, natural use in clinic or field setting. Perhaps the strongest value of the single case approaches are their inherent ability to comfort and address threats to internal validity and confounds.5 The history a participant brings to the study along with the changes as a function of being in the single case study are welcomed reactions (i.e., confounds to other types of studies). Further, the enhanced sensitivity of being in a study, recording behavior with an instrument in some settings, sets the participants’ behaviors to question. By contrast, in a single case study, these behaviors are implicitly manufactured with the design selection.


With focus currently on evidenced-based practice/medicine in health care, more exacting measurements of treatment are needed.6 The purposeful, visible emphasis on process and outcome date provides the participant/client with an ongoing view of the validity of the intervention approach. The evidence-based practice movement can be addressed easily with the low cost (i.e., low expense and time expenditure).

The Scientist Practitioner Model guides clinicians to use data collected in laboratory to guide decisions of treatment implementation. In turn, treatment implementation, according to the Scientist Practitioner Model,7 influences the direction of research. Research evidence of treatment effectiveness is valued. In single case designs, the systematic monitoring and evaluation positions the participant/client to adopt a problem-solving experiment, conjointly with the practitioner/field researcher on herself/himself. In so doing, the findings are immediate and directly applicable to their situation.1,4 In some cases, these approaches promote the generation of alternative interpretations of collected data and quite possibly, causal explanations of behaviors. The clinical practices of establishing support, setting up a conceptual/ethical concept of clients’ situation, identification of areas of change/strength/weakness, selection and implementation of treatment, evaluation of change/plan for relapse and follow-up are conducted within the simple case design context.2,5,6 All single case designs involve training the participant/client in observation practices. This training of the individual to understand what behavior to record, how to record the behavior and when to record the behavior is central to single case design. It is common for the practitioner/field researcher to work with the participant/client in their selection of a behavior to change and thereby, record.1,4 This becomes important as this descriptive data will ultimately become the pivotal source of information about the functional relationship between the target behavior for change and those behaviors that precede and follow (i.e., antecedent consequences) as they are typically interdependent. In fact, this circumstance is what is termed the functional assessment. Some representative target behaviors often used in an ABA single case design are: nutrition; hydration – amount of water; weight gain or loss; medication compliance; adherence to treatment; smoking cessation; substance use cessation. Client’s values are incorporated in the choice of targets and goal setting procedures. The baseline measurement is followed by the implementation of a change in a target behavior such as the examples listed above. After the implementation period, the participant/client returns to an adlib or no intervention schedule. The ABA design, like other single case designs, allows the client values to be incorporated into the choice of targets and goal setting procedures.4,5 In the AB design, the intervention is followed by a baseline period. The disadvantage to this approach of no comparison is offset by the ease of use, immediate implementation in clinical practice, and value of reactivity to measurement to the participant/client (i.e., their heightened awareness of their behavior as reflected in the measurement).1,6

The clinician in practice is apt to select the AB1B2, changing criterion design. The single case approach provides a means of measuring the increased amount of an intervention. For example, in Kazdin,2 increased expected levels of quiz performance are used across math objectives 1, 2, 3 and 4 as measured during daily school sessions.3,5

The purpose of a time series design is the periodic measurement of a participant/client. The design is represented as:

O1 O2 O3 O4 X O5 O6 O7 O8

where X represents the exposure of a participant/client to an experimental intervention. The O refers to a process of measurement in a temporal order.4,6 An example of the design’s use is in the area of sleep medicine where a daily sleep log measures minutes of sleep and minutes in bed rendering a sleep efficiency ratio that is examined each day. The intervention is often the implementation of a stimulus control procedure to reduce wake time after sleep onset and a designated sleep schedule of no napping and a regular wake-up time.


We find that single case designs are powerful measurement tools of behavior. The challenges and need for quantification that occurs in field settings can be uniquely and precisely addressed with single case designs.


1. Bloom M, Fischer J, Orne JE. Evaluating practice guidelines for the accountable professional. New York: Allyn and Bacon; 1996.

2. Kazdin AE. Research design in clinical psychology. 4th ed. New York: Macmillan; 2003.

3. Haynes SN, Heiby EM, eds. Behavioral assessment. Comprehensive handbook of psychological assessment. New York: Wiley; 2003.

4. Cook TD, Campbell DT. Quasi-experimentation: design and analysis for field settings. Chicago: Rand McNally; 1979.

5. Kennedy MM. Generalizing for single case studies. Evaluation Quarterly 1979;3:661-8.

6. Stout CE, Hayes RA. The evidence-based practice methods, models, and tools for mental health professionals. New York: Wiley; 2003.

7. Hays SC, Barlow DH, Nelson-Gray RO. The scientist practitioners: research and accountability in the age of managed care. 2nd ed. Boston: Allyn and Bacon; 1999.

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