Answer: Longitunidal Study.
Explanation: A longitundinal study is a research design in which the researcher studies and observes the behavior of the same variables (Could be people, traits, characteristics) over periods of time.
In this case, 1- Dr. Samuel wants to study relationship between variables 2- does not wish to establish a cause-effect relationship, but indeed, 3- he is conducting the study over a three year period with the same participants. Therefore, it is a Longitudinal Study.