Final answer:
Longitudinal and cross-sectional research are key to understanding changes in populations over time and at a single point in time, providing insights into developmental, health, educational, and societal issues.
Step-by-step explanation:
Longitudinal and cross-sectional research methodologies are often necessary to determine patterns and changes in populations over time or at a specific point in time. For example, in the field of public health, longitudinal studies may track a group of individuals over many years to understand risk factors for diseases, such as cancer in studies done by the American Cancer Society. Conversely, cross-sectional research might be used to compare different population segments at a single point in time, such as varying age groups' views on same-sex marriage. This particular approach can offer insights into different generational perspectives shaped by social climates. Both research designs offer valuable, albeit different, insights into human behavior and social phenomena, and are crucial to the exploration of developmental changes, disease progression, educational achievements, and societal trends.