1- An experimental design that compares two independent variables(A and B) to one another in the counterbalanced sequence A--B--B--A. Basic is the assumption that a change in the successive positional order of the two independent variables might affect the dependent so that the effect of the order in which the former are sequenced becomes apparent. Also known as ABBA order/sequence.
2- A general approach to organism as its own control (true experiment, within-subjects) research designs, based on works such as Murray Sidman's Tactics of scientific Research, in which the sequence of conditions of a study might be described eventually as A-B-C-B or A-B-A-B-BC-B or any other sequence that will show clearly the impact of conditions A (usually a baseline), B, and C on the dependent variable.