Which design collects data from the same individuals over time to observe changes?

• A. Cross-sectional design
• B. Experimental design
• C. Longitudinal design
• D. Case study

Answer: (C)

Longitudinal design follows the same individuals across multiple time points to observe how they change. By repeatedly measuring the same people, researchers can track development, aging, or shifts in attitudes within individuals, revealing patterns of change that would be invisible if you only looked at a single time point. This approach helps distinguish true intra-individual change from differences that exist between different people at one moment.

In contrast, cross-sectional designs collect data at one point in time from different individuals, so they show how groups differ at that moment but not how a given person changes over time. Experimental designs focus on manipulating a variable and observing the outcome under controlled conditions, which is about causality rather than tracking natural change over time. Case studies dive deep into one person or a small number of cases, offering rich detail but not necessarily demonstrating how changes unfold across a larger sample over time.