Final answer:
The number of tiers in a multiple baseline design represents the number of subjects or behaviors being observed.
Step-by-step explanation:
In a multiple baseline design, the number of tiers represents the number of subjects or behaviors being observed. Each tier represents a different subject or behavior.
For example, if you are conducting a study on the effectiveness of an intervention on reading skills for three students, you would have three tiers, with each tier representing a different student. Each tier would have its own baseline and intervention phase.
There is no specific or most common number of tiers in a multiple baseline design, as it depends on the specific research question and design of the study.