Although researchers often begin a research study intending to use an ABAB design, circumstances that develop during the study may require adding new treatments or modifying the sequence of baseline and treatment phases.
As a result, the exact sequence of baseline and treatment phases evolves during the course of the study creating an essentially unlimited number of potential designs.
For example, a researcher may plan for an ABAB design but switch to a new treatment (C) when the participant fails to respond to the first treatment phase, thus creating a more complex phase-change design.