Final answer:
The type of study where a single group is measured across time using two or more different treatments is called a longitudinal study.
Step-by-step explanation:
A study in which a single group is measured across time using two or more different treatments is known as a longitudinal study. This type of study involves following the same group of participants over an extended period of time, and it is particularly useful for observing changes and developments as they occur naturally over time. For instance, a study measuring the impact of diet on health at various points throughout a person's adulthood would be considered a longitudinal study.