Catalina Rey, PhD, BCBA-D and Paige O’Neill, MA, BCBA
University of Nebraska Medical Center’s Munroe-Meyer Institute
Imagine you have a child with autism who receives multiple services: speech therapy, ABA therapy, and dietary supplements. They have shown significant improvements in skills and other behavior, but you are unsure which service is responsible for their progress. One parent may credit the therapies whereas the other parent may say that all the progress is due to the supplements. Clinicians, families, and experienced researchers face this challenge everyday as they evaluate the effectiveness of treatments.
When an individual receives more than one treatment at a time, it can create multiple treatment interference. This occurs when the effect of one treatment is changed by the exposure of another treatment (Campbell & Stanley, 1963; Kazdin, 1982). Treatment A and Treatment B, for example, can influence one another, making it hard to determine their effects in isolation. For caregivers, this interference complicates decisions about which treatment to continue, change, or stop. Researchers may also struggle to identify the most effective treatment in a study when multiple interventions are in use. It is important to understand which specific treatment leads to positive changes (Frampton, 2024), but multiple treatment interference makes this difficult.
How Multiple Treatments May Impact Outcomes
Different treatments can interact in various ways. Sometimes, one treatment makes another treatment appear more effective than it is. For example, Luna is potty training at school and at ABA therapy. At school, the teacher conducts regular potty sits, where Luna is encouraged to sit on the toilet for a few minutes. If she voids in the toilet, great; if not, they move on with the day. In ABA therapy, Luna’s therapists use a more comprehensive approach, providing enthusiastic praise for successful voids and corrective feedback for accidents. If Luna learns to use the potty at school and ABA therapy, we may conclude that both approaches are equally effective. But it is possible Luna’s success at school is partly due to the more thorough approach within her ABA therapy sessions. Without the ABA intervention, she may not have learned to use the potty at school as quickly.
The opposite effect can also happen. Take Jose, for example. He receives an allowance for doing chores at his mom’s house, but at Dad’s house, he only gets praise. If Jose completes all his chores at Mom’s but not at Dad’s, we might assume praise alone is not enough to motivate him. However, praise may only appear less effective when compared to the allowance. If both parents used only praise, Jose may have done his chores just as well at both houses without a financial incentive.
Conclusion
Multiple treatment interference is possible whenever two or more treatments are provided to the same person simultaneously (Barlow & Hayes, 1979). The presence of more than one treatment can alter the effect that any single intervention would have in isolation. When it comes to interpreting research findings that involve multiple treatments, it is important to understand that you cannot necessarily apply one of the treatments and expect the same results (Kazdin, 1982). There is no easy way to control for multiple treatment interference, but providers and caregivers interested in evaluating therapeutic effects can consider introducing new treatments one at a time to better detect any added benefit. Furthermore, it is important for caregivers to be transparent about other current interventions in place so that the threat of multiple treatment interference can be considered properly. In fact, it would be beneficial for teams to keep careful records of the full array of interventions used, when key components are changed (e.g., an increase in dosage of a medication) and subsequent changes in treatment targets to help guide decisions about which treatment is most impactful.
References
Barlow, D. H., & Hayes, S. C. (1979). Alternating treatments design: One strategy for comparing the effects of two treatments in a single subject. Journal of Applied Behavior Analysis, 12(2), 199-210. https://doi.org/10.1901/jaba.1979.12-199
Campbell, D. T., & Stanley, J. C. (1963). Experimental and quasi-experimental designs for research. In N. L. Gage (Ed.), Handbook of research on teaching. Rand McNally.
Frampton, S. (2024). An overview of internal validity: Was it really the treatment that made a difference? Science in Autism Treatment, 21(08).
Kazdin, A. E. (1982). Single-case research designs: Methods for clinical and applied settings. Oxford University Press.
Citation for this article:
Rey, C. N., & O’Neill, P. (2024). Science Corner: Multiple treatment interference as a threat to internal validity. Science in Autism Treatment, 21(12).
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