Answer:
Cross-sectional
Step-by-step explanation:
cross-sectional research approach otherwise known as cross-sectional analysis is a type of observational study used for population-based surveys to analyze data from a population, or a representative subgroup, at a specific point in time. The participants in a cross-sectional study are just selected based on the involvement and exclusion criteria set for the study. Once the participants have been selected for the study, the investigator follows the study to assess the exposure and the outcomes. The above explanation of clearly indicates that Dr. McLean is clearly employing the cross sectional research approach as he his dealing with some certain subset of a population based on age.