Alternating Treatments Design

The alternating treatments design (ATD), also called the multielement design, is a single-case experimental method that rapidly alternates two or more conditions or treatments within a single individual to compare their effects directly. Rather than running each treatment in a long block, the analyst switches between conditions in close succession — often session by session — in a counterbalanced or randomized order so that time, sequence, and setting do not favor one condition. The signature of an effect is a consistent vertical separation between the data paths of the conditions: when one treatment's outcomes reliably sit above another's across the alternation, the design attributes the difference to the treatment rather than to extraneous variables. Because comparison is built into the rapid alternation, the ATD can demonstrate differential effects quickly and without withdrawing an effective treatment to baseline. It fits squarely within the single-case design standards formalized by Kratochwill and colleagues in 2013, which treat systematic manipulation and replicated demonstrations as the basis for experimental control.