Answer:
The answer is a longitudinal study.
Step-by-step explanation:
Longitudinal studies are carried out over a prolonged period of time, using the same subjects and analysing the same factors. These are mostly done to examine changes in behaviour, as well as medical studies determining health effects.
The greatest advantage is that it reflects accurate results, as they involve close, rigorous observation. However, longitudinal studies are naturally very time-consuming and could be expensive.