Which of the following is true of longitudinal studies?
A) They measure units from a sample of the population at only one point in time.
B) They are often described as "snapshots" of the population.
C) They include "sample surveys."
D) They are prevalent in marketing research, outnumbering cross-sectional and causal studies.
E) They repeatedly measure the same sample units of a population over a period of time.
Answer: E) They repeatedly measure the same sample units of a population over a period of time.