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304 PART IV: Applied ResearchFIGURE 9.5Examples of behavioral records showing possible relationships between baseline and interventionphases of a behavior modification program. The arrow indicates the start of an intervention.ABBaseline Intervention Baseline InterventionCDBehaviorBehaviorBehaviorBehaviorBaseline Intervention Baseline InterventionPanel C of Figure 9.5 illustrates another potential problem that can arisewhen baselines show an increasing or decreasing trend. If the goal of the interventionwas to increase frequency of behavior, the decreasing trend shown inpanel C poses no problem of interpretation. An intervention that reversed thedecreasing trend can be taken as evidence that the treatment was effective. Ifthe goal of the intervention was to reduce the frequency of a behavior, however,the problem would be more serious. This situation is illustrated in panel D.Here we see a decreasing trend in the baseline stage and continued reductionof frequency in the treatment stage. It would be difficult to know whether thetreatment had an effect because the decrease following the intervention couldbe due to the intervention or to a continuation of the baseline trend. When anintervention is expected to have an effect in the same direction as a baselinetrend, the change following the intervention must be much more marked thanthat shown in panel D to support a conclusion that the treatment had been effective(Kazdin, 1978). This problem becomes even more troubling because atreatment effect in a single-subject design is usually judged by visually inspectingthe behavioral record. It is often difficult to say what constitutes a “marked”change in the behavioral record (see, for example, Parsonson & Baer, 1992). It isan especially good idea in these circumstances to complement the observationsof the target behavior with other means of evaluation such as making comparisonswith “normal” individuals or asking for subjective evaluations from othersfamiliar with the individual.Questions of External Validity A frequent criticism of single-subject researchdesigns is that the findings have limited external validity. In other words, the