When using the changing-criterion single-case research design, what is a good rule of thumb for determining how much the criterion should change at each phase?

a. in the new phase, the participant should be required to increase performance by 50% relative to the previous phase.
b. the criterion change should be relatively large, to provide motivation.
c. the criterion change should be relatively small, so as not to discourage the participant.
d. the criterion change should be large enough to detect differences in behavior from the previous phase, but not so large as to be unattainable.

D