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Psychology (9,695)
PSYB01H3 (581)
Chapter 9

Chapter 9

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Department
Psychology
Course
PSYB01H3
Professor
Connie Boudens
Semester
Fall

Description
Chapter 9: Small-N and Single-Subject Designs  Single-subject designs (sometimes referred to as single-case designs) offer an alternative to group designs  The focus is on an N = 1, a single subject, although many researchers instead refer to this type of design as a “small-N design” and apply it to between one and nine subjects  Useful for research on interventions in behavior analysis and clinical practice, in which the focus is on a single individual FOUNDATIONS OF SMALL-N DESIGNS  A small-N design is “an in-depth study of a single or relatively few subjects under tightly controlled experimental conditions in which the independent variable(s) is repeatedly manipulated over successive trials or conditions and in which the dependent variable(s) is repeatedly measured”  Small-N and single-subject designs typically have four components: o Repeated measurement of the dependent variable; o Baseline phase; o Treatment phase(s), with all subjects exposed to each phase; o Graphic display, perhaps supplemented by statistical analysis. REPEATED MEASURE  Must be able to measure the subject’s status on the target problem at regular time intervals, whether the intervals are hours, days, weeks, months, or the like  Repeated measures of the dependent variable can begin when the client is receiving an intervention for other problems  Limitations should not preclude using the information if that is all that is available, particularly for evaluating practice o Using client records or questioning to significant members  Agencies often collect quite a bit of data about their operations, and these data can be used to obtain repeated measurements BASELINE PHASE (A)  Represents the period in which the intervention to be evaluated is not offered to the subject  Repeated measurements of the dependent variable are taken or reconstructed o Measures reflect the status of the client on the dependent variable prior to the implementation of the intervention  Provide two aspects of control analogous to a control group in a group design o In a group design, we expect the treatment group to have different scores than the control group after the intervention. In a single- subject design, the subject serves as the control as the repeated baseline measurements establish the pattern of scores that we expect the intervention to change o Control group design, random assignment controls for threats to internal validity. In a single-subject design, the repeated baseline measurements allow the researcher to discount some of the threats to the internal validity of the design PATTERNS  A stable line is a line that is relatively flat, with little variability in the scores so that the scores fall in a narrow band **REFER TO 9.2**  Trend, in which the scores may be either increasing or decreasing during the baseline period o May be linear, curvilinear (rate of change is accelerating over time), or cyclical (regular increases and decreases)  As a general rule, the more data points, the more certain you will be about the pattern; it takes at least three consecutive measures that fall in some pattern for you to have confidence in the shape of the baseline pattern INTERNAL VALIDITY  When repeated measurements are taken during the baseline phase, several threats to internal validity are controlled  Problems of maturation, instrumentation, statistical regression, and testing may be controlled by the repeated measurement because patterns illustrative of these threats to internal validity should appear in the baseline  When baseline measures are stable lines, these threats may be ruled out, but it is more difficult to rule out some threats if the pattern is a trend, particularly if the trend is in the desired direction  If statistical regression and testing effects occur, the impact is likely to appear initially in the baseline measures  A high score obtained from a measurement may be lower in a second measurement because of statistical regression or because of the respondent’s acclimation to the measurement process  If there were only one baseline measure, then the first intervention measure might reflect these effects o Multiple measures, the effect of statistical regression, if present should occur in the beginning of measurement, and continued measurement should produce a stable baseline pattern  Repeated measurement in a baseline will not control for an extraneous event event
(history) that occurs between the last baseline measurement and the first intervention measurement  Longer the time period between the two measurement points, the greater the possibility that an event might influence the subject’s scores TREATMENT PHASE (B)  Represents the time period during which the intervention is implemented  During the baseline phase, repeated measurements of the same dependent variable using the same measures are obtained  The patterns and magnitude of the data points will be compared to the data points in the baseline phase to determine whether a change has occurred  Tony Tripodi and David Barlow and Michel Hersen – recommend that the length of the treatment phase be as long as the baseline phase GRAPHING  Graphing the data facilitates monitoring and evaluating the impact of the intervention  Y-axis is used to represent the scores of the dependent variable, while the x- axis represents a unit of time, such as an hour, a day, a week, or month MEASURING TARGETS OF INTERVENTION  The dependent variable in a single-subject design is the concern or issue that is the focus of the intervention o This target for change may be one specific problem or different aspects of that problem  Once the target of the intervention has been identified, you must determine how you will operationalize the outcome  Measures of behaviors, status, or functioning are often characterized in four ways: o Frequency refers to counting the number of times a behavior occurs or the number of times people experience different feelings within a particular time period. Frequency counts are useful for measuring targets that happen regularly, but counting can be burdensome if the behavior occurs too often. On the other hand, if the behavior happens only periodically, the counts will not be meaningful. o Duration refers to the length of time an event or some symptom lasts and usually is measured for each occurrence of the event or symptom. A measure of duration requires fewer episodes than do frequency counts of the target problem. o Interval, or the length of time between events. This kind of measure may not be appropriate for events or symptoms that happen frequently unless the intent of the intervention is to delay their onset. o Magnitude or intensity of a particular behavior or psychology cal state can be measured. Magnitude or intensity measures are applied to psychological symptoms or attitudes such as measures of depressive symptoms, quality of peer interactions, or self-esteem.  Reliability and validity of the instruments should have been tested on subjects of the same age, gender, and ethnicity as the client who is the focus of the single-subject design  Problem in gathering the data is the issue of reactivity o Tripodi – suggests that changes due to reactivity may be short in duration and observable in the baseline, so repeated measurements in the baseline might mitigate this problem  Concern about measurement is the feasibility of the measurement process o Repeatedly taking measures can be cumbersome, inconvenient, and difficult  Choice of measurement must be sensitive enough to detect changes o Measuring device is too global, it may be impossible to detect incremental or small changes, particularly in such target problems as psychological status, feelings, emotions, and attitudes ANALYZING SMALL-N AND SINGLE-SUBJECT DESIGNS  How then can we use singlesubject designs to decide whether the intervention has been effective? o Visually examine the graphed data o Use a statistical technique such as the two-standard deviation-band, chi-square analysis, or time series to analyze the data  Regardless of whether you use visual inspection or one of these statistical approaches, the overriding issue is the practical (or clinical) significance of the findings  Practical significance at times is subjective, there are several principles you might apply to reduce the uncertainty o Setting criteria. One simple method is to establish with the client or community the criteria for success. If the intervention reaches that point, then the change is meaningful. o Cut-off scores. A second method, particularly useful for psychological symptoms, is whether the intervention has reduced the problem to a level below a clinical cut-off score o Costs and benefits. A third way to view practical significance is to weigh the costs and benefits to produce the change VISUAL ANALYSIS  The process of looking at a graph of the data points to determine whether the intervention has altered the subject’s preintervention pattern of scores  Three concepts that help guide visual inspection are level, trend, and variability LEVEL  Level of the phase scores may be summarized by drawing a line at the typical score for each phase separately  Level may be summarized into a single observation using the mean (the average of the observations in the phase) or the median (the value at which 50 % of the scores in the phase are higher and 50%
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