Answer:
longitudinal
Explanation:
Longitudinal study: In psychological research, the term "longitudinal study" is also referred as "panel study" or "longitudinal survey", and is described as a research or study design that encompasses distinct repeated observations of the same participants or variables by the researcher, investigator, or experimenter for either a long or short period in time.
In the question above, the given statement signifies that the professor conducted a longitudinal study.