Final answer:
A case study involves a psychologist observing one person over a period of time to understand their physical and psychological changes.
Step-by-step explanation:
The study design that involves a psychologist, often a therapist, observing one person over a period of time is called a case study. In a case study, the psychologist collects a great deal of information from one individual to better understand their physical and psychological changes over the lifespan. However, this approach is prone to researcher bias and it is difficult to generalize conclusions to the larger population.