Chapter 12 Quasi-Experimental and Single-Case Designs
Definition The quasi-experimental approach is used when it is impossible to randomly assign participants to comparison groups and when a researcher is faced with a situation where only one or two participants can participate in the research study (single case designs).
Reliability is weaker Causal explanations can be made when using quasi- experimental designs but only when you collect data that demonstrate that plausible rival explanations are unlikely, and the evidence will still not be as strong as with experimental
Three quasi-experimental designs The nonequivalent comparison-group design The interrupted time-series design The regression discontinuity design
Nonequivalent Comparison-Group Design Contains a treatment group and a nonequivalent untreated comparison group which are administered pretest and posttest measures. The groups are “nonequivalent” because you lack random assignment Only when you can rule out the effects of confounding variables can you confidently attribute the observed group difference at the posttest to the independent variable.
Example?
Interrupted Time-Series Design Compares pretest and posttest responses obtained from a single group of participants. Participants are pretested and posttested a number of times The pretesting phase is called the baseline A treatment effect is demonstrated only if post-treatment responses differs from the pretreatment responses. The main potentially confounding variable that cannot be ruled out is a history effect. The history threat is a plausible rival explanation if some event other than the treatment co- occurs with the onset of the treatment.
Example?
Regression Discontinuity Design Looks for a discontinuity in regression lines between individuals who score lower and higher than some predetermined cutoff score on an assignment variable. It is a design used to determine if individuals meeting some predetermined criteria profit from receiving some special treatment.
For example: Use a standardized test with a cutoff at 50 Administer treatment to those scoring in pretest above 50 Administer posttest to both groups, and see if there is a discontinuity in the regression line. If there is none, the treatment didn’t work. This is actually quite a strong design, and methodologists have, for a number of years, been trying to get researcher to use this design more frequently. Figure 12.9 on pg 340 clarifies
ControlTreatment 50 No discontinuity in scores
ControlTreatment 50 Regression Discontinuity
4 Single-Case Experimental Designs Treats single participants, one at a time. Or one single group of individuals. 1. A-B-A design, 2. A-B-A-B design, 3. Multiple-baseline design 4. Changing-criterion design
A-B-A and A-B-A-B Designs The A-B-A design - participant is repeatedly pretested (the first A phase), then the experimental treatment condition is administered and the participant is repeatedly post-tested (the B phase) Finally, the treatment is stopped and participant is tested again (second A phase) The effect of the treatment is demonstrated if the pattern differs from A to B then reverts back to A
For example, Student talks out of turn frequently (A) Teacher administers treatment (B) Teacher stops administering treatment (A) If student returns to talking out of turn frequently, treatment was effective. One limitation of the A-B-A design is that it ends with baseline condition at the end of the experiment. This limitation can be overcome by including a fourth phase which adds a second administration of the treatment condition so the design becomes an A-B-A-B design.
Multiple-Baseline Design This design focuses on two or more different behaviors in the same individual Or on the same behavior exhibited by two or more individuals Or on the same behavior exhibited by one individual but in different settings
Example: Same behavior in four people Step 1: Get baseline pretest on all four people Step 2: Treat first person only, don’t treat the other three Step 3: Treat second person only Step 4: Treat third person only Step 5: Treat fourth person only If results show change during treatment, treatment is effective
A polluting variable The other three may change behavior because one of the four is changing. For example, if all four talk out of turn, and one is silenced through treatment, the other three may grow quiet because one is silent, not because of the treatment
Changing-Criterion Design This is a single-case design that is used when a behavior needs to be shaped over time or when it is necessary to gradually change a behavior through successive treatment periods to reach a desired criterion. Step 1: Get baseline Step 2: Administer treatment Step 3: Administer stronger treatment Step 4: Administer even stronger treatment
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