Often overlooked in primary care research is the single subject research design. A single subject research design can be used to study the time course, variability, or effect of an intervention or treatment on a single patient. This experimental research design involving multiple measurements over time on a single subject (n = 1) has been labelled as a clinical trial of n = 1, a randomised clinical trial in a single patient, a within subject design, patient care study, A-B, or a single subject design.1–9
When conducting patient care, a physician is most probably treating the patient using a single subject paradigm. Typically, a patient presents to a physician with a complaint or symptom. As a result of the history, examination, and medical testing by the physician, for the patient’s presenting complaint, an intervention or treatment might be started. Evaluation is then made of the effectiveness of the treatment in eliminating or modifying the complaint or underlying identified condition. When conducting a single subject research design as compared with patient care, sound research methodology would need to be included. This would include standardisation of the intervention including the assessment of compliance, objective measurement of the outcomes, procedures to minimise bias such as blinding or second observers, and so forth. With the implementations, it can be argued that patient care is a special case of a single subject research design.
The randomised parallel group clinical trial design is usually considered as the gold standard in clinical research. The results of these trials provide information on the average treatment effects for the studied populations and serve as guidelines for evidence based medicine. At times, the results from these trials might not always be applicable in determining the most effective treatment for an individual patient: (1) patients might be different clinically from those that participated in the trial, (2) the heterogeneity of the patients in the trial may limit the generalisability of the results to particular patient populations, and (3) the strict criteria for trial participation might be of limited applicability to a general clinic patient populations. To examine the applicability of the results of a clinical trial to a specific patient, a single subject research design could be used.
An Institute of Medicine report10 has provided some guidelines as to the use of these small clinical trials. Specifically, warranted situations might include rare diseases, unique study populations, individually tailored treatments, isolated environments, emergency situations, and public health urgency.10 Practice based primary care research, commonly, encompasses individually tailored treatments (for example, glycaemic control), isolated environments (for example, rural health), and unique study populations (for example, HIV infected pregnant adolescent).
A patient care or an experimental single subject research design can have multiple periods of measurement and multiple times for intervention or treatment. At the onset of each of these designs, a series of baseline (A) observations are taken to assess the patient or subject in the initial or diseased state. A course of treatment is prescribed, denoted by the treatment or (B) phase. The patient or subject continues with the treatment, while assessing the effectiveness across time, using the same outcome variables that were used during the baseline phase. This describes the primary A-B single subject design. Other designs might include removal of the intervention or treatment referred to as washouts, the second baseline (A), a readministration of the treatment (B), or different treatments (C). The research question of interest should guide the single subject design used in terms of combinations of baselines and treatments (A-B, A-B-A, A-B-A-B, A-B-C, etc). Depending upon the research question of interest, varying combinations of phases of observation consisting of baseline, washout, intervention, and so forth.11 If more than one treatment is planned, the order of the treatments may be randomised; and, where feasible, the researcher and patient are blinded to the order of the treatments. Within each of these phases, multiple across time observations are obtained. If there are multiple repeating phases, these are considered as periods. One, or more than one, outcome variable(s) may be measured.
A WORKING EXAMPLE
As an example of the use of a single subject research design in clinical practice, a person with diabetes might present to their family physician for additional disease and treatment management. In the language of a single subject design, the initial reporting of the patient to their family physician, and the physician’s evaluation of the patient is considered the baseline phase. This phase, of a single subject design, encompasses an evaluation of the initial state of the patient. For this diabetic patient, this phase would consist of review of the past blood glucose readings and some initial testing (for example, HbA1C). For a single subject design, the period of time when the patient is receiving an intervention is referred to as the treatment phase. For this diabetic patient, that could include an insulin adjustment, a knowledge based diabetic education course, a behavioural intervention, and others. The assessment of the effectiveness of the treatment is then made by a comparison of the initial with the treatment phase.12 If the first treatment was not effective, a choice might be made to stop the initial treatment or change to a new treatment, or both, thus introducing another baseline or treatment phase into the design. During each of these phases, one or more measurements might be taken to assess the course or variability in the response, which entails multiple measurements of HbA1C at spaced visits. In a single subject design, the typical number of measurements within a phase is seven.9
Presented in figure 1 is an illustration of an implemented A-B single subject research design. The research question of interest for this study was whether a comprehensive intervention for diabetes management would be effective in lowering the fasting blood glucose values. In choosing the patient to participate in this single subject study, careful consideration would be given to identify a patient that would be considered as:
An illustration of an A-B single subject research design.
typical in terms of the practice demographics,
typical for the disease presentation and progression,
in need of lower fasting blood glucose values,
anticipated to be compliant for the treatment changes,
anticipated to be compliant for the necessary outcome assessments, and
would give informed consent.
For this particular single subject design, there are two phases to this design: a baseline (A) and a treatment (B). The baseline and treatment are both administered one time. There are measurements across time, within both the baseline and treatment phases, the patient was seen every two months for a fasting blood glucose measurement.
Presented in figure 1 are the data for the fasting blood glucose values. The patient was a 57 year old white woman, with an onset of insulin dependent diabetes in 1992. Presented are four measurements, every two months, before and after the intervention. A total of eight observations are presented, and each observation is measured and reported (for example, laboratory used, time of day, etc) in the same manner. The intervention consisted of a prescribed exercise regimen, weight management, and participation in a counselling session that was goaled to present diabetes management. As can be seen, it appears that the intervention was effective in lowering the measured fasting blood glucose in this subject. As in all single subject designs, the research question of interest should guide the single subject design used.
RECENT EXAMPLES FROM LITERATURE
Fisher, although most often associated with multiple subject designs, first introduced a single subject experimental paradigm in 1945.1 Since the introduction, the single subject design has been commonly used within the social and educational sciences.6 This design has recently been used as a means of investigation in medicine involving such areas as drug therapy,9 gastroenterology,13–15 internal medicine,15 paediatrics,16 family medicine,17 cardiology,18 nutrition,19 and others.9,16,20–22 During the 1980s, McMaster University established a service to direct and collaborate with physicians in planning and conducting n-of-1 trials.20 It is reported20 that of the 57 completed trials, 50 of those trials provided a definite clinical answer and in 15 cases the study results of the trial resulted in the physician changing the treatment of the patient.
A recent literature example by Langer et al16 reports on a single subject randomised trial to assess the effect of cisapride on symptoms arising from gastro-oesophageal reflux in paediatric patients. A placebo and cisapride phase were studied, with three study periods (A-B-A-B-A-B). The outcome variables of interest were number of episodes per five days for vomiting, gagging, and stools. In addition, Guyatt et al22 report on a single subject study of a randomised controlled investigation of theophylline. Two study periods of drug and placebo phases were used (A-B-A-B). The symptoms of shortness of breath, the need for an inhaler, and sleep disturbance as patient reported on a seven point scale were used as the outcomes.
At times, single subject research designs have advantages over more traditional group based designs. Some advantages outlined by Franklin et al25 that are especially applicable to practice based family medicine research, include: (1) research situations where research funds are scarce, especially for professionals working in private practice or small clinical settings; (2) research questions that aim to study the process of change; and (3) research questions that are driven by clinical work with the crucial question as to whether a treatment would work for a particular patient. The utilisation of the single subject design for the example above showed these (that is, the data presented in fig 1). This design permitted the investigation of the process of change, both within the baseline and after intervention phases and across these phases, with limited research funds in a private physician office, with the overall intention of answering the research question as to whether this treatment would work for this particular patient.
Limitations of the single subject research design are generalisability of the study conclusions and the methodological and statistical assumptions that are typically needed for inferential statistical tests.4,23 A single subject design provides limited support for conclusions regarding populations of subjects. The results of a single subject design may provide positive findings of the effectiveness of an intervention for a particular subject, however the portion of the population that would show this effectiveness and the size of the benefit remain unknown. The non-violation of the methodological and statistical assumptions that are typically needed for inferential tests are difficult to evaluate and test when using a single subject design, because of the limited available data. The evaluation of the validity of the assumptions is more difficult to assess in these small samples. Even with these limitations, estimates and the tests of the effectiveness or intervention effect on the studied subject can be accurately and validly tested using a single subject design.
POSSIBLE RESEARCH QUESTIONS FOR THE PRIMARY CARE PHYSICIAN
Depending upon the interest of their clinical and patient care needs, the number of research questions that can be addressed by a primary care physician in the practice setting is quite broad. A few examples of possible research questions that can be answered using a single subject research design include:
How does the introduction of a specific antihypertensive drug affect the systolic blood pressure for this patient?
Is there a change in compliance for this patient with twice daily blood glucose readings with the introduction of a reminder system?
What is the optimal dose of a specific antidepressant drug for the control of this patient’s self reported symptoms and obtained scores on the Beck depression scale?
Based upon the reported low back and leg pain scores for this patient, does the introduction of an exercise regimen have an impact on those scores?
Does the introduction of an email contact reduce the missed appointments for a particular patient?
Is the level of clinical effectiveness obtained for this particular patient comparable to the results of a recently published clinical trial for the effectiveness of acid suppressive therapy for the control of dyspepsia?
With group based research designs, internal and external validity issues need to be considered and balanced.26 As the primary questions for a single subject research design concern the investigation of the process of change and whether a treatment would work for a particular patient, internal validity (elimination of bias) issues are paramount. Unfortunately, because of the nature of these designs, external validity (generalisability) when balanced against internal validity is typically left with limited control.
The generalisability of the results from a single subject research study is limited. Possible means of increasing the external validity, generalisability of the results, is by: (1) choosing a subject that is representative of the general type of patients for which this intervention would be used6 and (2) by conducting replication studies involving variation in researchers, subjects, or practices.27
When planning to implement a single subject design, the specifications necessary to conduct a multiple subject randomised clinical trial must also be followed. The planning phase must incorporate forethought in the choice of the:
implementation of the treatment,
number of phases,
number of periods, and
number of observations.
Some guidelines have been prepared for the design of single subject research studies.4,8,9,16,24 Given the research question of interest, a single subject design should be considered as a means of investigation for practice based primary care research.
What Is Single-Subject Research?
Single-subject research is a type of quantitative research that involves studying in detail the behavior of each of a small number of participants. Note that the term single-subject does not mean that only one participant is studied; it is more typical for there to be somewhere between two and 10 participants. (This is why single-subject research designs are sometimes called small-n designs, where n is the statistical symbol for the sample size.) Single-subject research can be contrasted with group research, which typically involves studying large numbers of participants and examining their behavior primarily in terms of group means, standard deviations, and so on. The majority of this book is devoted to understanding group research, which is the most common approach in psychology. But single-subject research is an important alternative, and it is the primary approach in some areas of psychology.
Before continuing, it is important to distinguish single-subject research from two other approaches, both of which involve studying in detail a small number of participants. One is qualitative research, which focuses on understanding people’s subjective experience by collecting relatively unstructured data (e.g., detailed interviews) and analyzing those data using narrative rather than quantitative techniques. Single-subject research, in contrast, focuses on understanding objective behavior through experimental manipulation and control, collecting highly structured data, and analyzing those data quantitatively.
It is also important to distinguish single-subject research from case studies. A case study is a detailed description of an individual, which can include both qualitative and quantitative analyses. (Case studies that include only qualitative analyses can be considered a type of qualitative research.) The history of psychology is filled with influential cases studies, such as Sigmund Freud’s description of “Anna O.” (see Note 10.5 “The Case of “Anna O.””) and John Watson and Rosalie Rayner’s description of Little Albert (Watson & Rayner, 1920), who learned to fear a white rat—along with other furry objects—when the researchers made a loud noise while he was playing with the rat. Case studies can be useful for suggesting new research questions and for illustrating general principles. They can also help researchers understand rare phenomena, such as the effects of damage to a specific part of the human brain. As a general rule, however, case studies cannot substitute for carefully designed group or single-subject research studies. One reason is that case studies usually do not allow researchers to determine whether specific events are causally related, or even related at all. For example, if a patient is described in a case study as having been sexually abused as a child and then as having developed an eating disorder as a teenager, there is no way to determine whether these two events had anything to do with each other. A second reason is that an individual case can always be unusual in some way and therefore be unrepresentative of people more generally. Thus case studies have serious problems with both internal and external validity.
The Case of “Anna O.”
Sigmund Freud used the case of a young woman he called “Anna O.” to illustrate many principles of his theory of psychoanalysis (Freud, 1961). (Her real name was Bertha Pappenheim, and she was an early feminist who went on to make important contributions to the field of social work.) Anna had come to Freud’s colleague Josef Breuer around 1880 with a variety of odd physical and psychological symptoms. One of them was that for several weeks she was unable to drink any fluids. According to Freud,
But according to Freud, a breakthrough came one day while Anna was under hypnosis.
Freud’s interpretation was that Anna had repressed the memory of this incident along with the emotion that it triggered and that this was what had caused her inability to drink. Furthermore, her recollection of the incident, along with her expression of the emotion she had repressed, caused the symptom to go away.
As an illustration of Freud’s theory, the case study of Anna O. is quite effective. As evidence for the theory, however, it is essentially worthless. The description provides no way of knowing whether Anna had really repressed the memory of the dog drinking from the glass, whether this repression had caused her inability to drink, or whether recalling this “trauma” relieved the symptom. It is also unclear from this case study how typical or atypical Anna’s experience was.
“Anna O.” was the subject of a famous case study used by Freud to illustrate the principles of psychoanalysis.
Assumptions of Single-Subject Research
Again, single-subject research involves studying a small number of participants and focusing intensively on the behavior of each one. But why take this approach instead of the group approach? There are several important assumptions underlying single-subject research, and it will help to consider them now.
First and foremost is the assumption that it is important to focus intensively on the behavior of individual participants. One reason for this is that group research can hide individual differences and generate results that do not represent the behavior of any individual. For example, a treatment that has a positive effect for half the people exposed to it but a negative effect for the other half would, on average, appear to have no effect at all. Single-subject research, however, would likely reveal these individual differences. A second reason to focus intensively on individuals is that sometimes it is the behavior of a particular individual that is primarily of interest. A school psychologist, for example, might be interested in changing the behavior of a particular disruptive student. Although previous published research (both single-subject and group research) is likely to provide some guidance on how to do this, conducting a study on this student would be more direct and probably more effective.
A second assumption of single-subject research is that it is important to discover causal relationships through the manipulation of an independent variable, the careful measurement of a dependent variable, and the control of extraneous variables. For this reason, single-subject research is often considered a type of experimental research with good internal validity. Recall, for example, that Hall and his colleagues measured their dependent variable (studying) many times—first under a no-treatment control condition, then under a treatment condition (positive teacher attention), and then again under the control condition. Because there was a clear increase in studying when the treatment was introduced, a decrease when it was removed, and an increase when it was reintroduced, there is little doubt that the treatment was the cause of the improvement.
A third assumption of single-subject research is that it is important to study strong and consistent effects that have biological or social importance. Applied researchers, in particular, are interested in treatments that have substantial effects on important behaviors and that can be implemented reliably in the real-world contexts in which they occur. This is sometimes referred to as social validity (Wolf, 1976). The study by Hall and his colleagues, for example, had good social validity because it showed strong and consistent effects of positive teacher attention on a behavior that is of obvious importance to teachers, parents, and students. Furthermore, the teachers found the treatment easy to implement, even in their often chaotic elementary school classrooms.
Who Uses Single-Subject Research?
Single-subject research has been around as long as the field of psychology itself. In the late 1800s, one of psychology’s founders, Wilhelm Wundt, studied sensation and consciousness by focusing intensively on each of a small number of research participants. Herman Ebbinghaus’s research on memory and Ivan Pavlov’s research on classical conditioning are other early examples, both of which are still described in almost every introductory psychology textbook.
In the middle of the 20th century, B. F. Skinner clarified many of the assumptions underlying single-subject research and refined many of its techniques (Skinner, 1938). He and other researchers then used it to describe how rewards, punishments, and other external factors affect behavior over time. This work was carried out primarily using nonhuman subjects—mostly rats and pigeons. This approach, which Skinner called the experimental analysis of behavior—remains an important subfield of psychology and continues to rely almost exclusively on single-subject research. For excellent examples of this work, look at any issue of the Journal of the Experimental Analysis of Behavior. By the 1960s, many researchers were interested in using this approach to conduct applied research primarily with humans—a subfield now called applied behavior analysis (Baer, Wolf, & Risley, 1968). Applied behavior analysis plays an especially important role in contemporary research on developmental disabilities, education, organizational behavior, and health, among many other areas. Excellent examples of this work (including the study by Hall and his colleagues) can be found in the Journal of Applied Behavior Analysis.
Although most contemporary single-subject research is conducted from the behavioral perspective, it can in principle be used to address questions framed in terms of any theoretical perspective. For example, a studying technique based on cognitive principles of learning and memory could be evaluated by testing it on individual high school students using the single-subject approach. The single-subject approach can also be used by clinicians who take any theoretical perspective—behavioral, cognitive, psychodynamic, or humanistic—to study processes of therapeutic change with individual clients and to document their clients’ improvement (Kazdin, 1982).
- Single-subject research—which involves testing a small number of participants and focusing intensively on the behavior of each individual—is an important alternative to group research in psychology.
- Single-subject studies must be distinguished from case studies, in which an individual case is described in detail. Case studies can be useful for generating new research questions, for studying rare phenomena, and for illustrating general principles. However, they cannot substitute for carefully controlled experimental or correlational studies because they are low in internal and external validity.
- Single-subject research has been around since the beginning of the field of psychology. Today it is most strongly associated with the behavioral theoretical perspective, but it can in principle be used to study behavior from any perspective.
- Practice: Find and read a published article in psychology that reports new single-subject research. (A good source of articles published in the Journal of Applied Behavior Analysis can be found at http://seab.envmed.rochester.edu/jaba/jabaMostPop-2011.html.) Write a short summary of the study.
Practice: Find and read a published case study in psychology. (Use case study as a key term in a PsycINFO search.) Then do the following:
- Describe one problem related to internal validity.
- Describe one problem related to external validity.
- Generate one hypothesis suggested by the case study that might be interesting to test in a systematic single-subject or group study.
Baer, D. M., Wolf, M. M., & Risley, T. R. (1968). Some current dimensions of applied behavior analysis. Journal of Applied Behavior Analysis, 1, 91–97.
Freud, S. (1961). Five lectures on psycho-analysis. New York, NY: Norton.
Kazdin, A. E. (1982). Single-case research designs: Methods for clinical and applied settings. New York, NY: Oxford University Press.
Skinner, B. F. (1938). The behavior of organisms: An experimental analysis. New York, NY: Appleton-Century-Crofts.
Watson, J. B., & Rayner, R. (1920). Conditioned emotional reactions. Journal of Experimental Psychology, 3, 1–14.
Wolf, M. (1976). Social validity: The case for subjective measurement or how applied behavior analysis is finding its heart. Journal of Applied Behavior Analysis, 11, 203–214.
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