Applied hybrid single-case experiments published between 2016 and 2020: A systematic review

Single-case experimental designs (SCEDs) are frequently used research designs in psychology, (special) education, and related fields. Hybrid designs are formed by combining two or more of the basic SCED forms (i.e. phase designs, alternation designs, multiple baseline designs, and changing criterion designs). Hybrid designs have the potential to tackle complex research questions and increase internal validity, but relatively little is known about their use in actual research practice. Therefore, we systematically reviewed SCED hybrid designs published between 2016 and 2020. The systematic review of 67 studies indicates that a hybrid of phase designs and multiple baseline designs is most popular. Hybrid designs are most frequently analyzed by means of visual analysis paired with descriptive statistics. Randomization in the study design is common only for one particular kind of hybrid design. Examples of hybrid studies reveal that these designs are particularly popular in educational research. We compare some of the results of the systematic review to those obtained by Hammond and Gast, Shadish and Sullivan, and Tanious and Onghena. Finally, we discuss the results of the present systematic review in light of the need for specific guidelines for hybrid designs, including analytical methods, design specific randomization and reporting, and the need for terminological clarification.